Storing hazardous material in a subterranean formation

ABSTRACT

Methods for storing or disposing of nuclear waste include forming a drillhole that extends into the Earth from a terranean surface. The drillhole includes an entry at least proximate the terranean surface, a substantially vertical drillhole portion, and a hazardous material storage drillhole portion that is coupled to the substantially vertical drillhole portion and is formed in a subterranean salt formation. The methods further include moving a storage canister into the hazardous material storage drillhole portion. The storage canister is sized to fit from the drillhole entry through the substantially vertical drillhole portion, and into the hazardous material storage drillhole portion of the drillhole. The storage canister has an inner cavity that encloses nuclear waste material. The methods further include positioning a seal in the drillhole to isolate the hazardous material storage drillhole portion of the drillhole from the entry of the drillhole.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation application of and claims priorityunder 35 U.S.C. § 120 to U.S. application Ser. No. 17/401,411, filed onAug. 13, 2021, which will issue as U.S. Pat. No. 11,338,337, which is adivisional of, and claims priority to, U.S. patent application Ser. No.17/145,505, filed on Jan. 11, 2021, now U.S. Pat. No. 11,135,629, whichis a continuation of, and claims priority under 35 U.S.C. § 120 to, U.S.patent application Ser. No. 16/419,866, filed on May 22, 2019, now U.S.Pat. No. 10,926,306, which is a continuation of, and claims priority to,U.S. patent application Ser. No. 15/997,819, filed on Jun. 5, 2018, nowU.S. Pat. No. 10,300,512, and entitled “STORING HAZARDOUS MATERIAL IN ASUBTERRANEAN FORMATION,” which in turn claims priority under 35 U.S.C. §119 to U.S. Provisional Patent Application Ser. No. 62/515,050, filed onJun. 5, 2017, and entitled “STORING HAZARDOUS MATERIAL IN A SUBTERRANEANFORMATION.” The entire contents of the previous applications areincorporated by reference herein.

TECHNICAL FIELD

This disclosure relates to storing hazardous material in a subterraneanformation and, more particularly, storing spent nuclear fuel in asubterranean formation.

BACKGROUND

Hazardous waste is often placed in long-term, permanent, orsemi-permanent storage so as to prevent health issues among a populationliving near the stored waste. Such hazardous waste storage is oftenchallenging, for example, in terms of storage location identificationand surety of containment. For instance, the safe storage of nuclearwaste (e.g., spent nuclear fuel, whether from commercial power reactors,test reactors, or even high-grade military waste) is considered to beone of the outstanding challenges of energy technology. Safe storage ofthe long-lived radioactive waste is a major impediment to the adoptionof nuclear power in the United States and around the world. Conventionalwaste storage methods have emphasized the use of tunnels, and isexemplified by the design of the Yucca Mountain storage facility. Othertechniques include boreholes, including vertical boreholes, drilled intocrystalline basement rock. Other conventional techniques include forminga tunnel with boreholes emanating from the walls of the tunnel inshallow formations to allow human access.

SUMMARY

In a general implementation, a hazardous material storage repositoryincludes a drillhole extending into the Earth and including an entry atleast proximate a terranean surface, the drillhole including asubstantially vertical drillhole portion, a transition drillhole portioncoupled to the substantially vertical drillhole portion, and a hazardousmaterial storage drillhole portion coupled to the transition drillholeportion, at least one of the transition drillhole portion or thehazardous material storage drillhole portion including an isolationdrillhole portion that is directed vertically toward the terraneansurface and away from an intersection between the substantially verticaldrillhole portion and the transition drillhole portion; a storagecanister positioned in the hazardous material storage drillhole portion,the storage canister sized to fit from the drillhole entry through thesubstantially vertical drillhole portion, the transition drillholeportion, and into the hazardous material storage drillhole portion ofthe drillhole, the storage canister including an inner cavity sizedenclose hazardous material; and a seal positioned in the drillhole, theseal isolating the hazardous material storage drillhole portion of thedrillhole from the entry of the drillhole.

In an aspect combinable with the general implementation, the isolationdrillhole portion includes a vertically inclined drillhole portion thatincludes a proximate end coupled to the transition drillhole portion ata first depth and a distal end opposite the proximate end at a seconddepth shallower than the first depth.

In another aspect combinable with any of the previous aspects, thevertically inclined drillhole portion includes the hazardous materialstorage drillhole portion.

In another aspect combinable with any of the previous aspects, aninclination angle of the vertically inclined drillhole portion isdetermined based at least in part on a distance associated with adisturbed zone of a geologic formation that surrounds the verticallyinclined drillhole portion and a length of a distance tangent to alowest portion of the storage canister and the substantially verticaldrillhole portion.

In another aspect combinable with any of the previous aspects, thedistance associated with the disturbed zone of the geologic formationincludes a distance between an outer circumference of the disturbed zoneand a radial centerline of the vertically inclined drillhole portion.

In another aspect combinable with any of the previous aspects, theinclination angle is about 3 degrees.

In another aspect combinable with any of the previous aspects, theisolation drillhole portion includes a J-section drillhole portioncoupled between the substantially vertical drillhole portion and thehazardous material storage drillhole portion.

In another aspect combinable with any of the previous aspects, theJ-section drillhole portion includes the transition drillhole portion.

In another aspect combinable with any of the previous aspects, thehazardous material storage drillhole portion includes at least one of asubstantially horizontal drillhole portion or a vertically inclineddrillhole portion.

In another aspect combinable with any of the previous aspects, theisolation drillhole portion includes a vertically undulating drillholeportion coupled to the transition drillhole portion.

In another aspect combinable with any of the previous aspects, thetransition drillhole portion includes a curved drillhole portion betweenthe substantially vertical drillhole portion and the verticallyundulating drillhole portion.

In another aspect combinable with any of the previous aspects, thehazardous material storage drillhole portion is located within or belowa barrier layer that includes at least one of a shale formation layer, asalt formation layer, or other impermeable formation layer.

In another aspect combinable with any of the previous aspects, thehazardous material storage drillhole portion is vertically isolated, bythe barrier layer, from a subterranean zone that includes mobile water.

In another aspect combinable with any of the previous aspects, thehazardous material storage drillhole portion is formed below the barrierlayer and is vertically isolated from the subterranean zone thatincludes mobile water by the barrier layer.

In another aspect combinable with any of the previous aspects, thehazardous material storage drillhole portion is formed within thebarrier layer, and is vertically isolated from the subterranean zonethat includes mobile water by at least a portion of the barrier layer.

In another aspect combinable with any of the previous aspects, thebarrier layer includes a permeability of less than about 0.01millidarcys.

In another aspect combinable with any of the previous aspects, thebarrier layer includes a brittleness of less than about 10 MPa, wherebrittleness includes a ratio of compressive stress of the barrier layerto tensile strength of the barrier layer.

In another aspect combinable with any of the previous aspects, thebarrier layer includes a thickness proximate the hazardous materialstorage drillhole portion of at least about 100 feet.

In another aspect combinable with any of the previous aspects, thebarrier layer includes a thickness proximate the hazardous materialstorage drillhole portion that inhibits diffusion of the hazardousmaterial that escapes the storage canister through the barrier layer foran amount of time that is based on a half-life of the hazardousmaterial.

In another aspect combinable with any of the previous aspects, thebarrier layer includes about 20 to 30% weight by volume of clay ororganic matter.

In another aspect combinable with any of the previous aspects, thebarrier layer includes an impermeable layer.

In another aspect combinable with any of the previous aspects, thebarrier layer includes a leakage barrier defined by a time constant forleakage of the hazardous material of 10,000 years or more.

In another aspect combinable with any of the previous aspects, thebarrier layer includes a hydrocarbon or carbon dioxide bearingformation.

In another aspect combinable with any of the previous aspects, thehazardous material includes spent nuclear fuel.

Another aspect combinable with any of the previous aspects furtherincludes at least one casing assembly that extends from at or proximatethe terranean surface, through the drillhole, and into the hazardousmaterial storage drillhole portion.

In another aspect combinable with any of the previous aspects, thestorage canister includes a connecting portion configured to couple toat least one of a downhole tool string or another storage canister.

In another aspect combinable with any of the previous aspects, theisolation drillhole portion includes a spiral drillhole.

In another aspect combinable with any of the previous aspects, theisolation drillhole portion has a specified geometry independent of astress state of a rock formation into which the isolation drillholeportion is formed.

In another general implementation, a method for storing hazardousmaterial includes moving a storage canister through an entry of adrillhole that extends into a terranean surface, the entry at leastproximate the terranean surface, the storage canister including an innercavity sized enclose hazardous material; moving the storage canisterthrough the drillhole that includes a substantially vertical drillholeportion, a transition drillhole portion coupled to the substantiallyvertical drillhole portion, and a hazardous material storage drillholeportion coupled to the transition drillhole portion, at least one of thetransition drillhole portion or the hazardous material storage drillholeportion including an isolation drillhole portion that is directedvertically toward the terranean surface and away from an intersectionbetween the substantially vertical drillhole portion and the transitiondrillhole portion; moving the storage canister into the hazardousmaterial storage drillhole portion; and forming a seal in the drillholethat isolates the storage portion of the drillhole from the entry of thedrillhole.

In an aspect combinable with the general implementation, the isolationdrillhole portion includes a vertically inclined drillhole portion thatincludes a proximate end coupled to the transition drillhole portion ata first depth and a distal end opposite the proximate end at a seconddepth shallower than the first depth.

In another aspect combinable with any of the previous aspects, thevertically inclined drillhole portion includes the hazardous materialstorage drillhole portion.

In another aspect combinable with any of the previous aspects, aninclination angle of the vertically inclined drillhole portion isdetermined based at least in part on a distance associated with adisturbed zone of a geologic formation that surrounds the verticallyinclined drillhole portion and a length of a distance tangent to alowest portion of the storage canister and the substantially verticaldrillhole portion.

In another aspect combinable with any of the previous aspects, thedistance associated with the disturbed zone of the geologic formationincludes a distance between an outer circumference of the disturbed zoneand a radial centerline of the vertically inclined drillhole portion.

In another aspect combinable with any of the previous aspects, theinclination angle is about 3 degrees.

In another aspect combinable with any of the previous aspects, theisolation drillhole portion includes a J-section drillhole portioncoupled between the substantially vertical drillhole portion and thehazardous material storage drillhole portion.

In another aspect combinable with any of the previous aspects, theJ-section drillhole portion includes the transition drillhole portion.

In another aspect combinable with any of the previous aspects, thehazardous material storage drillhole portion includes at least one of asubstantially horizontal drillhole portion or a vertically inclineddrillhole portion.

In another aspect combinable with any of the previous aspects, theisolation drillhole portion includes a vertically undulating drillholeportion coupled to the transition drillhole portion.

In another aspect combinable with any of the previous aspects, thetransition drillhole portion includes a curved drillhole portion betweenthe substantially vertical drillhole portion and the verticallyundulating drillhole portion.

In another aspect combinable with any of the previous aspects, thehazardous material storage drillhole portion is located within or belowa barrier layer that includes at least one of a shale formation layer, asalt formation layer, or other impermeable formation layer.

In another aspect combinable with any of the previous aspects, thehazardous material storage drillhole portion is vertically isolated, bythe barrier layer, from a subterranean zone that includes mobile water.

In another aspect combinable with any of the previous aspects, thehazardous material storage drillhole portion is formed below the barrierlayer and is vertically isolated from the subterranean zone thatincludes mobile water by the barrier layer.

In another aspect combinable with any of the previous aspects, thehazardous material storage drillhole portion is formed within thebarrier layer, and is vertically isolated from the subterranean zonethat includes mobile water by at least a portion of the barrier layer.

In another aspect combinable with any of the previous aspects, thebarrier layer includes a permeability of less than about 0.01millidarcys.

In another aspect combinable with any of the previous aspects, thebarrier layer includes a brittleness of less than about 10 MPa, wherebrittleness includes a ratio of compressive stress of the barrier layerto tensile strength of the barrier layer.

In another aspect combinable with any of the previous aspects, thebarrier layer includes a thickness proximate the hazardous materialstorage drillhole portion of at least about 100 feet.

In another aspect combinable with any of the previous aspects, thebarrier layer includes a thickness proximate the hazardous materialstorage drillhole portion that inhibits diffusion of the hazardousmaterial that escapes the storage canister through the barrier layer foran amount of time that is based on a half-life of the hazardousmaterial.

In another aspect combinable with any of the previous aspects, thebarrier layer includes about 20 to 30% weight by volume of clay ororganic matter.

In another aspect combinable with any of the previous aspects, thebarrier layer includes an impermeable layer.

In another aspect combinable with any of the previous aspects, thebarrier layer includes a leakage barrier defined by a time constant forleakage of the hazardous material of 10,000 years or more.

In another aspect combinable with any of the previous aspects, thebarrier layer includes a hydrocarbon or carbon dioxide bearingformation.

In another aspect combinable with any of the previous aspects, thehazardous material includes spent nuclear fuel.

Another aspect combinable with any of the previous aspects furtherincludes at least one casing assembly that extends from at or proximatethe terranean surface, through the drillhole, and into the hazardousmaterial storage drillhole portion.

In another aspect combinable with any of the previous aspects, thestorage canister includes a connecting portion configured to couple toat least one of a downhole tool string or another storage canister.

Another aspect combinable with any of the previous aspects furtherincludes prior to moving the storage canister through the entry of thedrillhole that extends into the terranean surface, forming the drillholefrom the terranean surface to a subterranean formation.

Another aspect combinable with any of the previous aspects furtherincludes installing a casing in the drillhole that extends from at orproximate the terranean surface, through the drillhole, and into thehazardous material storage drillhole portion.

Another aspect combinable with any of the previous aspects furtherincludes cementing the casing to the drillhole.

Another aspect combinable with any of the previous aspects furtherincludes, subsequent to forming the drillhole, producing hydrocarbonfluid from the subterranean formation, through the drillhole, and to theterranean surface.

Another aspect combinable with any of the previous aspects furtherincludes removing the seal from the drillhole; and retrieving thestorage canister from the hazardous material storage drillhole portionto the terranean surface.

Another aspect combinable with any of the previous aspects furtherincludes monitoring at least one variable associated with the storagecanister from a sensor positioned proximate the hazardous materialstorage drillhole portion; and recording the monitored variable at theterranean surface.

In another aspect combinable with any of the previous aspects, themonitored variable includes at least one of radiation level,temperature, pressure, presence of oxygen, presence of water vapor,presence of liquid water, acidity, or seismic activity.

Another aspect combinable with any of the previous aspects furtherincludes based on the monitored variable exceeding a threshold valueremoving the seal from the drillhole; and retrieving the storagecanister from the hazardous material storage drillhole portion to theterranean surface.

In another general implementation, a method for storing hazardousmaterial includes moving a storage canister through an entry of adrillhole that extends into a terranean surface, the entry at leastproximate the terranean surface, the storage canister including an innercavity sized enclose hazardous material; moving the storage canisterthrough the drillhole that includes a substantially vertical drillholeportion, a transition drillhole portion coupled to the substantiallyvertical drillhole portion, and a hazardous material storage drillholeportion coupled to the transition drillhole portion, the hazardousmaterial storage drillhole portion located below a self-healinggeological formation, the hazardous material storage drillhole portionvertically isolated, by the self-healing geological formation, from asubterranean zone that includes mobile water; moving the storagecanister into the hazardous material storage drillhole portion; andforming a seal in the drillhole that isolates the storage portion of thedrillhole from the entry of the drillhole.

In an aspect combinable with the general implementation, theself-healing geologic formation includes at least one of shale, salt,clay, or dolomite.

In another general implementation, a hazardous material storagerepository includes a drillhole extending into the Earth and includingan entry at least proximate a terranean surface, the drillhole includinga substantially vertical drillhole portion, a transition drillholeportion coupled to the substantially vertical drillhole portion, and ahazardous material storage drillhole portion coupled to the transitiondrillhole portion, the hazardous material storage drillhole portionlocated below a self-healing geological formation, the hazardousmaterial storage drillhole portion vertically isolated, by theself-healing geological formation, from a subterranean zone thatincludes mobile water; a storage canister positioned in the hazardousmaterial storage drillhole portion, the storage canister sized to fitfrom the drillhole entry through the substantially vertical drillholeportion, the transition drillhole portion, and into the hazardousmaterial storage drillhole portion of the drillhole, the storagecanister including an inner cavity sized enclose hazardous material; anda seal positioned in the drillhole, the seal isolating the hazardousmaterial storage drillhole portion of the drillhole from the entry ofthe drillhole.

In an aspect combinable with the general implementation, theself-healing geologic formation includes at least one of shale, salt,clay, or dolomite.

Implementations of a hazardous material storage repository according tothe present disclosure may include one or more of the followingfeatures. For example, a hazardous material storage repository accordingto the present disclosure may allow for multiple levels of containmentof hazardous material within a storage repository located thousands offeet underground, decoupled from any nearby mobile water. A hazardousmaterial storage repository according to the present disclosure may alsouse proven techniques (e.g., drilling) to create or form a storage areafor the hazardous material, in a subterranean zone proven to havefluidly sealed hydrocarbons therein for millions of years. As anotherexample, a hazardous material storage repository according to thepresent disclosure may provide long-term (e.g., thousands of years)storage for hazardous material (e.g., radioactive waste) in a shaleformation that has geologic properties suitable for such storage,including low permeability, thickness, and ductility, among others. Inaddition, a greater volume of hazardous material may be stored at lowcost—relative to conventional storage techniques—due in part todirectional drilling techniques that facilitate long horizontalboreholes, often exceeding a mile in length. In addition, rockformations that have geologic properties suitable for such storage maybe found in close proximity to sites at which hazardous material may befound or generated, thereby reducing dangers associated withtransporting such hazardous material.

Implementations of a hazardous material storage repository according tothe present disclosure may also include one or more of the followingfeatures. Large storage volumes, in turn, allow for the storage ofhazardous materials to be emplaced without a need for complex priortreatment, such as concentration or transfer to different forms orcanisters. As a further example, in the case of nuclear waste materialfrom a reactor for instance, the waste can be kept in its originalpellets, unmodified, or in its original fuel rods, or in its originalfuel assemblies, which contain dozens of fuel rods. In another aspect,the hazardous material may be kept in an original holder but a cement orother material is injected into the holder to fill the gaps between thehazardous materials and the structure. For example, if the hazardousmaterial is stored in fuel rods which are, in turn, stored in fuelassemblies, then the spaces between the rods (typically filled withwater when inside a nuclear reactor) could be filled with cement orother material to provide yet an additional layer of isolation from theoutside world. As yet a further example, secure and low cost storage ofhazardous material is facilitated while still permitting retrieval ofsuch material if circumstances deem it advantageous to recover thestored materials.

The details of one or more implementations of the subject matterdescribed in this disclosure are set forth in the accompanying drawingsand the description below. Other features, aspects, and advantages ofthe subject matter will become apparent from the description, thedrawings, and the claims.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A is a schematic illustration of an example implementation of ahazardous material storage repository system during a deposit orretrieval operation according to the present disclosure.

FIG. 1B is a schematic illustration of a portion of the exampleimplementation of the hazardous material storage repository system ofFIG. 1A that shows an example determination of a minimum angle of aninclined portion of the hazardous material storage repository system.

FIG. 2 is a schematic illustration of another example implementation ofa hazardous material storage repository system during a deposit orretrieval operation according to the present disclosure.

FIG. 3 is a schematic illustration of another example implementation ofa hazardous material storage repository system during a deposit orretrieval operation according to the present disclosure.

FIG. 4A-4C are schematic illustrations of other example implementationsof a hazardous material storage repository system according to thepresent disclosure.

FIG. 5A is a top view, and FIGS. 5B-5C are side views, of schematicillustrations of another example implementation of a hazardous materialstorage repository system.

DETAILED DESCRIPTION

FIG. 1A is a schematic illustration of example implementations of ahazardous material storage repository system, e.g., a subterraneanlocation for the long-term (e.g., tens, hundreds, or thousands of yearsor more) but retrievable safe and secure storage of hazardous material,during a deposit or retrieval operation according to the presentdisclosure. For example, turning to FIG. 1A, this figure illustrates anexample hazardous material storage repository system 100 during adeposit (or retrieval, as described below) process, e.g., duringdeployment of one or more canisters of hazardous material in asubterranean formation. As illustrated, the hazardous material storagerepository system 100 includes a drillhole 104 formed (e.g., drilled orotherwise) from a terranean surface 102 and through multiplesubterranean layers 112, 114, 116, and 132. Although the terraneansurface 102 is illustrated as a land surface, terranean surface 102 maybe a sub-sea or other underwater surface, such as a lake or an oceanfloor or other surface under a body of water. Thus, the presentdisclosure contemplates that the drillhole 104 may be formed under abody of water from a drilling location on or proximate the body ofwater.

The illustrated drillhole 104 is a directional drillhole in this exampleof hazardous material storage repository system 100. For instance, thedrillhole 104 includes a substantially vertical portion 106 coupled to aradiussed or curved portion 108, which in turn is coupled to an inclinedportion 110. As used in the present disclosure, “substantially” in thecontext of a drillhole orientation, refers to drillholes that may not beexactly vertical (e.g., exactly perpendicular to the terranean surface102) or exactly horizontal (e.g., exactly parallel to the terraneansurface 102), or exactly inclined at a particular incline angle relativeto the terranean surface 102. In other words, vertical drillholes oftenundulate offset from a true vertical direction, that they might bedrilled at an angle that deviates from true vertical, and inclineddrillholes often undulate offset from a true incline angle. Further, insome aspects, an inclined drillhole may not have or exhibit an exactlyuniform incline (e.g., in degrees) over a length of the drillhole.Instead, the incline of the drillhole may vary over its length (e.g., by1-5 degrees). As illustrated in this example, the three portions of thedrillhole 104—the vertical portion 106, the radiussed portion 108, andthe inclined portion 110—form a continuous drillhole 104 that extendsinto the Earth.

The illustrated drillhole 104, in this example, has a surface casing 120positioned and set around the drillhole 104 from the terranean surface102 into a particular depth in the Earth. For example, the surfacecasing 120 may be a relatively large-diameter tubular member (or stringof members) set (e.g., cemented) around the drillhole 104 in a shallowformation. As used herein, “tubular” may refer to a member that has acircular cross-section, elliptical cross-section, or other shapedcross-section. For example, in this implementation of the hazardousmaterial storage repository system 100, the surface casing 120 extendsfrom the terranean surface through a surface layer 112. The surfacelayer 112, in this example, is a geologic layer comprised of one or morelayered rock formations. In some aspects, the surface layer 112 in thisexample may or may not include freshwater aquifers, salt water or brinesources, or other sources of mobile water (e.g., water that movesthrough a geologic formation). In some aspects, the surface casing 120may isolate the drillhole 104 from such mobile water, and may alsoprovide a hanging location for other casing strings to be installed inthe drillhole 104. Further, although not shown, a conductor casing maybe set above the surface casing 120 (e.g., between the surface casing120 and the surface 102 and within the surface layer 112) to preventdrilling fluids from escaping into the surface layer 112.

As illustrated, a production casing 122 is positioned and set within thedrillhole 104 downhole of the surface casing 120. Although termed a“production” casing, in this example, the casing 122 may or may not havebeen subject to hydrocarbon production operations. Thus, the casing 122refers to and includes any form of tubular member that is set (e.g.,cemented) in the drillhole 104 downhole of the surface casing 120. Insome examples of the hazardous material storage repository system 100,the production casing 122 may begin at an end of the radiussed portion108 and extend throughout the inclined portion 110. The casing 122 couldalso extend into the radiussed portion 108 and into the vertical portion106.

As shown, cement 130 is positioned (e.g., pumped) around the casings 120and 122 in an annulus between the casings 120 and 122 and the drillhole104. The cement 130, for example, may secure the casings 120 and 122(and any other casings or liners of the drillhole 104) through thesubterranean layers under the terranean surface 102. In some aspects,the cement 130 may be installed along the entire length of the casings(e.g., casings 120 and 122 and any other casings), or the cement 130could be used along certain portions of the casings if adequate for aparticular drillhole 102. The cement 130 can also provide an additionallayer of confinement for the hazardous material in canisters 126.

The drillhole 104 and associated casings 120 and 122 may be formed withvarious example dimensions and at various example depths (e.g., truevertical depth, or TVD). For instance, a conductor casing (not shown)may extend down to about 120 feet TVD, with a diameter of between about28 in. and 60 in. The surface casing 120 may extend down to about 2500feet TVD, with a diameter of between about 22 in. and 48 in. Anintermediate casing (not shown) between the surface casing 120 andproduction casing 122 may extend down to about 8000 feet TVD, with adiameter of between about 16 in. and 36 in. The production casing 122may extend inclinedly (e.g., to case the inclined portion 110) with adiameter of between about 11 in. and 22 in. The foregoing dimensions aremerely provided as examples and other dimensions (e.g., diameters, TVDs,lengths) are contemplated by the present disclosure. For example,diameters and TVDs may depend on the particular geological compositionof one or more of the multiple subterranean layers (112, 114, 116, and132), particular drilling techniques, as well as a size, shape, ordesign of a hazardous material canister 126 that contains hazardousmaterial to be deposited in the hazardous material storage repositorysystem 100. In some alternative examples, the production casing 122 (orother casing in the drillhole 104) could be circular in cross-section,elliptical in cross-section, or some other shape.

As illustrated, the vertical portion 106 of the drillhole 104 extendsthrough subterranean layers 112, 114, 116, and 132, and, in thisexample, lands in a subterranean layer 119. As discussed above, thesurface layer 112 may or may not include mobile water. Subterraneanlayer 114, which is below the surface layer 112, in this example, is amobile water layer 114. For instance, mobile water layer 114 may includeone or more sources of mobile water, such as freshwater aquifers, saltwater or brine, or other source of mobile water. In this example ofhazardous material storage repository system 100, mobile water may bewater that moves through a subterranean layer based on a pressuredifferential across all or a part of the subterranean layer. Forexample, the mobile water layer 114 may be a permeable geologicformation in which water freely moves (e.g., due to pressure differencesor otherwise) within the layer 114. In some aspects, the mobile waterlayer 114 may be a primary source of human-consumable water in aparticular geographic area. Examples of rock formations of which themobile water layer 114 may be composed include porous sandstones andlimestones, among other formations.

Other illustrated layers, such as the impermeable layer 116 and thestorage layer 119, may include immobile water. Immobile water, in someaspects, is water (e.g., fresh, salt, brine), that is not fit for humanor animal consumption, or both. Immobile water, in some aspects, may bewater that, by its motion through the layers 116 or 119 (or both),cannot reach the mobile water layer 114, terranean surface 102, or both,within 10,000 years or more (such as to 1,000,000 years).

Below the mobile water layer 114, in this example implementation ofhazardous material storage repository system 100, is an impermeablelayer 116. The impermeable layer 116, in this example, may not allowmobile water to pass through. Thus, relative to the mobile water layer114, the impermeable layer 116 may have low permeability, e.g., on theorder of nanodarcy permeability. Additionally, in this example, theimpermeable layer 116 may be a relatively non-ductile (i.e., brittle)geologic formation. One measure of non-ductility is brittleness, whichis the ratio of compressive stress to tensile strength. In someexamples, the brittleness of the impermeable layer 116 may be betweenabout 20 MPa and 40 MPa.

As shown in this example, the impermeable layer 116 is shallower (e.g.,closer to the terranean surface 102) than the storage layer 119. In thisexample rock formations of which the impermeable layer 116 may becomposed include, for example, certain kinds of sandstone, mudstone,clay, and slate that exhibit permeability and brittleness properties asdescribed above. In alternative examples, the impermeable layer 116 maybe deeper (e.g., further from the terranean surface 102) than thestorage layer 119. In such alternative examples, the impermeable layer116 may be composed of an igneous rock, such as granite.

Below the impermeable layer 116 is the storage layer 119. The storagelayer 119, in this example, may be chosen as the landing for theinclined portion 110, which stores the hazardous material, for severalreasons. Relative to the impermeable layer 116 or other layers, thestorage layer 119 may be thick, e.g., between about 100 and 200 feet oftotal vertical thickness. Thickness of the storage layer 119 may allowfor easier landing and directional drilling, thereby allowing theinclined portion 110 to be readily emplaced within the storage layer 119during constructions (e.g., drilling). If formed through an approximatehorizontal center of the storage layer 119, the inclined portion 110 maybe surrounded by about 50 to 100 feet of the geologic formation thatcomprises the storage layer 119. Further, the storage layer 119 may alsohave only immobile water, e.g., due to a very low permeability of thelayer 119 (e.g., on the order of milli-or nanodarcys). In addition, thestorage layer 119 may have sufficient ductility, such that a brittlenessof the rock formation that comprises the layer 119 is between about 3MPa and 10 MPa. Examples of rock formations of which the storage layer119 may be composed include: shale and anhydrite. Further, in someaspects, hazardous material may be stored below the storage layer, evenin a permeable formation such as sandstone or limestone, if the storagelayer is of sufficient geologic properties to isolate the permeablelayer from the mobile water layer 114.

In some examples implementations of the hazardous material storagerepository system 100, the storage layer 119 (and/or the impermeablelayer 116) is composed of shale. Shale, in some examples, may haveproperties that fit within those described above for the storage layer119. For example, shale formations may be suitable for a long-termconfinement of hazardous material (e.g., in the hazardous materialcanisters 126), and for their isolation from mobile water layer 114(e.g., aquifers) and the terranean surface 102. Shale formations may befound relatively deep in the Earth, typically 3000 feet or greater, andplaced in isolation below any fresh water aquifers. Other formations mayinclude salt or other impermeable formation layer.

Shale formations (or salt or other impermeable formation layers), forinstance, may include geologic properties that enhance the long-term(e.g., thousands of years) isolation of material. Such properties, forinstance, have been illustrated through the long term storage (e.g.,tens of millions of years) of hydrocarbon fluids (e.g., gas, liquid,mixed phase fluid) without escape of substantial fractions of suchfluids into surrounding layers (e.g., mobile water layer 114). Indeed,shale has been shown to hold natural gas for millions of years or more,giving it a proven capability for long-term storage of hazardousmaterial. Example shale formations (e.g., Marcellus, Eagle Ford,Barnett, and otherwise) has stratification that contains many redundantsealing layers that have been effective in preventing movement of water,oil, and gas for millions of years, lacks mobile water, and can beexpected (e.g., based on geological considerations) to seal hazardousmaterial (e.g., fluids or solids) for thousands of years after deposit.

In some aspects, the formation of the storage layer 119 and/or theimpermeable layer 116 may form a leakage barrier, or barrier layer tofluid leakage that may be determined, at least in part, by the evidenceof the storage capacity of the layer for hydrocarbons or other fluids(e.g., carbon dioxide) for hundreds of years, thousands of years, tensof thousands of years, hundreds of thousands of years, or even millionsof years. For example, the barrier layer of the storage layer 119 and/orimpermeable layer 116 may be defined by a time constant for leakage ofthe hazardous material more than 10,000 years (such as between about10,000 years and 1,000,000 years) based on such evidence of hydrocarbonor other fluid storage.

Shale (or salt or other impermeable layer) formations may also be at asuitable depth, e.g., between 3000 and 12,000 feet TVD. Such depths aretypically below ground water aquifer (e.g., surface layer 112 and/ormobile water layer 114). Further, the presence of soluble elements inshale, including salt, and the absence of these same elements in aquiferlayers, demonstrates a fluid isolation between shale and the aquiferlayers.

Another particular quality of shale that may advantageously lend itselfto hazardous material storage is its clay content, which, in someaspects, provides a measure of ductility greater than that found inother, impermeable rock formations (e.g., impermeable layer 116). Forexample, shale may be stratified, made up of thinly alternating layersof clays (e.g., between about 20-30% clay by volume) and other minerals.Such a composition may make shale less brittle and, thus lesssusceptible to fracturing (e.g., naturally or otherwise) as compared torock formations in the impermeable layer (e.g., dolomite or otherwise).For example, rock formations in the impermeable layer 116 may havesuitable permeability for the long term storage of hazardous material,but are too brittle and commonly are fractured. Thus, such formationsmay not have sufficient sealing qualities (as evidenced through theirgeologic properties) for the long term storage of hazardous material.

The present disclosure contemplates that there may be many other layersbetween or among the illustrated subterranean layers 112, 114, 116, and119. For example, there may be repeating patterns (e.g., vertically), ofone or more of the mobile water layer 114, impermeable layer 116, andstorage layer 119. Further, in some instances, the storage layer 119 maybe directly adjacent (e.g., vertically) the mobile water layer 114,i.e., without an intervening impermeable layer 116. In some examples,all or portions of the radiussed drillhole 108 and the inclineddrillhole 110 may be formed below the storage layer 119, such that thestorage layer 119 (e.g., shale or other geologic formation withcharacteristics as described herein) is vertically positioned betweenthe inclined drillhole 110 and the mobile water layer 114.

Further, in this example implementation, a self-healing layer 132 may befound below the terranean surface 102 and between, for example, thesurface 102 and one or both of the impermeable layer 116 and the storagelayer 119. In some aspects, the self-healing layer 132 may comprise ageologic formation that can stop or impede a flow of hazardous material(whether in liquid, solid, or gaseous form) from a storage portion ofthe drillhole 104 to or toward the terranean surface 102. For example,during formation of the drillhole 104 (e.g., drilling), all are portionsof the geologic formations of the layers 112, 114, 116, and 119, may bedamaged (as illustrated by a damaged zone 140), thereby affecting orchanging their geologic characteristics (e.g., permeability). Indeed,although damaged zone 140 is illustrated between layers 114 and 132 forsimplicity sake, the damaged zone 140 may surround an entire length(vertical, curved, and inclined portions) of the drillhole 104 aparticular distance into the layers 112, 114, 116, 119, 132, andotherwise.

In certain aspects, the location of the drillhole 104 may be selected soas to be formed through the self-healing layer 132. For example, asshown, the drillhole 104 may be formed such that at least a portion ofthe vertical portion 106 of the drillhole 104 is formed to pass throughthe self-healing layer 132. In some aspects, the self-healing layer 132comprises a geologic formation that that does not sustain cracks forextended time durations even after being drilled therethrough. Examplesof the geologic formation in the self-healing layer 132 include clay ordolomite. Cracks in such rock formations tend to heal, that is, theydisappear rapidly with time due to the relative ductility of thematerial, and the enormous pressures that occur underground from theweight of the overlying rock on the formation in the self-healing layer.In addition to providing a “healing mechanism” for cracks that occur dueto the formation of the drillhole 104 (e.g., drilling or otherwise), theself-healing layer 132 may also provide a barrier to natural faults andother cracks that otherwise could provide a pathway for hazardousmaterial leakage (e.g., fluid or solid) from the storage region (e.g.,in the inclined portion 110) to the terranean surface 102, the mobilewater layer 114, or both.

As shown in this example, the inclined portion 110 of the drillhole 104includes a storage area 117 in a distal part of the portion 110 intowhich hazardous material may be retrievably placed for long-termstorage. For example, as shown, a work string 124 (e.g., tubing, coiledtubing, wireline, or otherwise) may be extended into the cased drillhole104 to place one or more (three shown but there may be more or less)hazardous material canisters 126 into long term, but in some aspects,retrievable, storage in the portion 110. For example, in theimplementation shown in FIG. 1A, the work string 124 may include adownhole tool 128 that couples to the canister 126, and with each tripinto the drillhole 104, the downhole tool 128 may deposit a particularhazardous material canister 126 in the inclined portion 110.

The downhole tool 128 may couple to the canister 126 by, in someaspects, a threaded connection or other type of connection, such as alatched connection. In alternative aspects, the downhole tool 128 maycouple to the canister 126 with an interlocking latch, such thatrotation (or linear movement or electric or hydraulic switches) of thedownhole tool 128 may latch to (or unlatch from) the canister 126. Inalternative aspects, the downhole tool 124 may include one or moremagnets (e.g., rare Earth magnets, electromagnets, a combinationthereof, or otherwise) which attractingly couple to the canister 126. Insome examples, the canister 126 may also include one or more magnets(e.g., rare Earth magnets, electromagnets, a combination thereof, orotherwise) of an opposite polarity as the magnets on the downhole tool124. In some examples, the canister 126 may be made from or include aferrous or other material attractable to the magnets of the downholetool 124.

As another example, each canister 126 may be positioned within thedrillhole 104 by a drillhole tractor (e.g., on a wireline or otherwise),which may push or pull the canister into the inclined portion 110through motorized (e.g., electric) motion. As yet another example, eachcanister 126 may include or be mounted to rollers (e.g., wheels), sothat the downhole tool 124 may push the canister 126 into the caseddrillhole 104.

In some example implementations, the canister 126, one or more of thedrillhole casings 120 and 122, or both, may be coated with afriction-reducing coating prior to the deposit operation. For example,by applying a coating (e.g., petroleum-based product, resin, ceramic, orotherwise) to the canister 126 and/or drillhole casings, the canister126 may be more easily moved through the cased drillhole 104 into theinclined portion 110. In some aspects, only a portion of the drillholecasings may be coated. For example, in some aspects, the substantiallyvertical portion 106 may not be coated, but the radiussed portion 108 orthe inclined portion 110, or both, may be coated to facilitate easierdeposit and retrieval of the canister 126.

FIG. 1A also illustrates an example of a retrieval operation ofhazardous material in the inclined portion 110 of the drillhole 104. Aretrieval operation may be the opposite of a deposit operation, suchthat the downhole tool 124 (e.g., a fishing tool) may be run into thedrillhole 104, coupled to the last-deposited canister 126 (e.g.,threadingly, latched, by magnet, or otherwise), and pull the canister126 to the terranean surface 102. Multiple retrieval trips may be madeby the downhole tool 124 in order to retrieve multiple canisters fromthe inclined portion 110 of the drillhole 104.

Each canister 126 may enclose hazardous material. Such hazardousmaterial, in some examples, may be biological or chemical waste or otherbiological or chemical hazardous material. In some examples, thehazardous material may include nuclear material, such as spent nuclearfuel recovered from a nuclear reactor (e.g., commercial power or testreactor) or military nuclear material. For example, a gigawatt nuclearplant may produce 30 tons of spent nuclear fuel per year. The density ofthat fuel is typically close to 10 (10 gm/cm³=10 kg/liter), so that thevolume for a year of nuclear waste is about 3 m³. Spent nuclear fuel, inthe form of nuclear fuel pellets, may be taken from the reactor and notmodified. Nuclear fuel pellet are solid, although they can contain andemit a variety of radioactive gases including tritium (13 yearhalf-life), krypton-85 (10.8 year half-life), and carbon dioxidecontaining C-14 (5730 year half-life).

In some aspects, the storage layer 119 should be able to contain anyradioactive output (e.g., gases) within the layer 119, even if suchoutput escapes the canisters 126. For example, the storage layer 119 maybe selected based on diffusion times of radioactive output through thelayer 119. For example, a minimum diffusion time of radioactive outputescaping the storage layer 119 may be set at, for example, fifty times ahalf-life for any particular component of the nuclear fuel pellets.Fifty half-lives as a minimum diffusion time would reduce an amount ofradioactive output by a factor of 1×10¹⁵. As another example, setting aminimum diffusion time to thirty half-lives would reduce an amount ofradioactive output by a factor of one billion.

For example, plutonium-239 is often considered a dangerous waste productin spent nuclear fuel because of its long half-life of 24,100 years. Forthis isotope, 50 half-lives would be 1.2 million years. Plutonium-239has low solubility in water, is not volatile, and as a solid. itsdiffusion time is exceedingly small (e.g., many millions of years)through a matrix of the rock formation that comprises the illustratedstorage layer 119 (e.g., shale or other formation). The storage layer119, for example comprised of shale, may offer the capability to havesuch isolation times (e.g., millions of years) as shown by thegeological history of containing gaseous hydrocarbons (e.g., methane andotherwise) for several million years. In contrast, in conventionalnuclear material storage methods, there was a danger that some plutoniummight dissolve in a layer that comprised mobile ground water uponconfinement escape.

As further shown in FIG. 1A, the storage canisters 126 may be positionedfor long term storage in the inclined portion 110, which, as shown, istilted upward at a small angle (e.g., 2-5 degrees) as it gets furtheraway from the vertical portion 106 of the drillhole 104. As illustrated,the inclined portion 110 tilts upward toward the terranean surface 102.In some aspects, for example when there is radioactive hazardousmaterial stored in the canisters 126, the inclination of the drillholeportion 110 may provide a further degree of safety and containment toprevent or impede the material, even if leaked from the canister 126,from reaching, e.g., the mobile water layer 114, the vertical portion106 of the drillhole 104, the terranean surface 102, or a combinationthereof. For example, radionuclides of concern in the hazardous materialtend to be relatively buoyant or heavy (as compared to brine or otherfluids that might fill the drillhole). Buoyant radionuclides may be thegreatest concern for leakage, since heavy elements and molecules tend tosink, and would not diffuse upward towards the terranean surface 102.Krypton gas, and particularly ¹⁴CO₂ (where ¹⁴C refers to radiocarbon,also called C-14, which is an isotope of carbon with a half-life of 5730years), is a buoyant radioactive element that is heavier than air (asare most gases) but much lighter than water. Thus, should ¹⁴CO₂ beintroduced into a water bath, such gas would tend to float upwardtowards the terranean surface 102. Iodine, on the other hand, is denserthan water, and would tend to diffuse downward if introduced into awater bath.

By including the inclined portion 110 of the drillhole 104, any suchdiffusion of radioactive material (e.g., even if leaked from a canister126 and in the presence of water or other liquid in the drillhole 104 orotherwise) would be directed angularly upward toward a distal end 121 ofthe inclined portion 110 and away from the radiussed portion 108 (andthe vertical portion 106) of the drillhole 104. Thus, leaked hazardousmaterial, even in a diffusible gas form, would not be offered a path(e.g., directly) to the terranean surface 102 (or the mobile water layer114) through the vertical portion 106 of the drillhole 110. Forinstance, the leaked hazardous material (especially in gaseous form)would be directed and gathered at the distal end 121 of the drillholeportion 110.

Alternative methods of depositing the canisters 126 into the inclineddrillhole portion 110 may also be implemented. For instance, a fluid(e.g., liquid or gas) may be circulated through the drillhole 104 tofluidly push the canisters 126 into the inclined drillhole portion 110.In some example, each canister 126 may be fluidly pushed separately. Inalternative aspects, two or more canisters 126 may be fluidly pushed,simultaneously, through the drillhole 104 for deposit into the inclinedportion 110. The fluid can be, in some cases, water. Other examplesinclude a drilling mud or drilling foam. In some examples, a gas may beused to push the canisters 126 into the drillhole, such as air, argon,or nitrogen.

In some aspects, the choice of fluid may depend at least in part on aviscosity of the fluid. For example, a fluid may be chosen with enoughviscosity to impede the drop of the canister 126 into the substantiallyvertical portion 106. This resistance or impedance may provide a safetyfactor against a sudden drop of the canister 126. The fluid may alsoprovide lubrication to reduce a sliding friction between the canister126 and the casings 120 and 122. The canister 126 can be conveyed withina casing filled with a liquid of controlled viscosity, density, andlubricant qualities. The fluid-filled annulus between the inner diameterof the casings 120 and 122 and the outer diameter of the conveyedcanister 126 represents an opening designed to dampen any high rate ofcanister motion, providing automatic passive protection in an unlikelydecoupling of the conveyed canister 126.

In some aspects, other techniques may be employed to facilitate depositof the canister 126 into the inclined portion 110. For example, one ormore of the installed casings (e.g., casings 120 and 122) may have railsto guide the storage canister 126 into the drillhole 102 while reducingfriction between the casings and the canister 126. The storage canister126 and the casings (or the rails) may be made of materials that slideeasily against one another. The casings may have a surface that iseasily lubricated, or one that is self-lubricating when subjected to theweight of the storage canister 126.

The fluid may also be used for retrieval of the canister 126. Forexample, in an example retrieval operation, a volume within the casings120 and 122 may be filled with a compressed gas (e.g., air, nitrogen,argon, or otherwise). As the pressure increases at an end of theinclined portion 110, the canisters 126 may be pushed toward theradiussed portion 108, and subsequently through the substantiallyvertical portion 106 to the terranean surface.

In some aspects, the drillhole 104 may be formed for the primary purposeof long-term storage of hazardous materials. In alternative aspects, thedrillhole 104 may have been previously formed for the primary purpose ofhydrocarbon production (e.g., oil, gas). For example, storage layer 119may be a hydrocarbon bearing formation from which hydrocarbons wereproduced into the drillhole 104 and to the terranean surface 102. Insome aspects, the storage layer 119 may have been hydraulicallyfractured prior to hydrocarbon production. Further in some aspects, theproduction casing 122 may have been perforated prior to hydraulicfracturing. In such aspects, the production casing 122 may be patched(e.g., cemented) to repair any holes made from the perforating processprior to a deposit operation of hazardous material. In addition, anycracks or openings in the cement between the casing and the drillholecan also be filled at that time.

For example, in the case of spent nuclear fuel as a hazardous material,the drillhole may be formed at a particular location, e.g., near anuclear power plant, as a new drillhole provided that the location alsoincludes an appropriate storage layer 119, such as a shale formation.Alternatively, an existing well that has already produced shale gas, orone that was abandoned as “dry,” (e.g., with sufficiently low organicsthat the gas in place is too low for commercial development), may beselected as the drillhole 104. In some aspects, prior hydraulicfracturing of the storage layer 119 through the drillhole 104 may makelittle difference in the hazardous material storage capability of thedrillhole 104. But such a prior activity may also confirm the ability ofthe storage layer 119 to store gases and other fluids for millions ofyears. If, therefore, the hazardous material or output of the hazardousmaterial (e.g., radioactive gasses or otherwise) were to escape from thecanister 126 and enter the fractured formation of the storage layer 119,such fractures may allow that material to spread relatively rapidly overa distance comparable in size to that of the fractures. In some aspects,the drillhole 102 may have been drilled for a production ofhydrocarbons, but production of such hydrocarbons had failed, e.g.,because the storage layer 119 comprised a rock formation (e.g., shale orotherwise) that was too ductile and difficult to fracture forproduction, but was advantageously ductile for the long-term storage ofhazardous material.

FIG. 1B is a schematic illustration of a portion of the exampleimplementation of the hazardous material storage repository system 100that shows an example determination of a minimum angle of the inclinedportion 110 of the hazardous material storage repository system 100. Forexample, as shown in system 100, the inclined portion 110 provides thatany path that leaking hazardous material (e.g., from one or more of thecanister 126) takes to the terranean surface 102 through the drillhole104 includes at least one downward component. In this case, the inclinedportion 110 is the downward component. In other example implementationsdescribed later, such as systems 200 and 300, other portions (e.g., aJ-section portion or undulating portion) may include at least onedownward component. Such paths, as shown in this example, dip below ahorizontal escape limit line 175 that intersects a canister 126 that isclosest (when positioned in the storage area 117) to the verticalportion 106 of the drillhole 104. and therefore must include a downwardcomponent.

In some aspects, an angle, a, of the inclined portion 110 of thedrillhole 104 may be determined (and thereby guide the formation of thedrillhole 104) according to a radius, R, of the damaged zone 140 of thedrillhole 104 and a distance, D, from the canister 126 that is closestto the vertical portion 106 of the drillhole 104. As shown in thecallout bubble in FIG. 1B, with knowledge of the distances R and D (orat least estimates), then the angle, a, can be computed according to thearctangent of R/D. In an example implementation, R may be about 1 meterwhile D may be about 20 meters. The angle, a, therefore, as thearctangent of R/D is about 3°. This is just one example of thedetermination of the angle, a, of a downward component (e.g., theinclined portion 110) of the drillhole 104 to ensure that such adownward component dips below the horizontal escape limit line 175.

FIG. 2 is a schematic illustration of example implementations of anotherhazardous material storage repository system, e.g., a subterraneanlocation for the long-term (e.g., tens, hundreds, or thousands of yearsor more) but retrievable safe and secure storage of hazardous material,during a deposit or retrieval operation according to the presentdisclosure. For example, turning to FIG. 2 , this figure illustrates anexample hazardous material storage repository system 200 during adeposit (or retrieval, as described below) process, e.g., duringdeployment of one or more canisters of hazardous material in asubterranean formation. As illustrated, the hazardous material storagerepository system 200 includes a drillhole 204 formed (e.g., drilled orotherwise) from a terranean surface 202 and through multiplesubterranean layers 212, 214, and 216. Although the terranean surface202 is illustrated as a land surface, terranean surface 202 may be asub-sea or other underwater surface, such as a lake or an ocean floor orother surface under a body of water. Thus, the present disclosurecontemplates that the drillhole 204 may be formed under a body of waterfrom a drilling location on or proximate the body of water.

The illustrated drillhole 204 is a directional drillhole in this exampleof hazardous material storage repository system 200. For instance, thedrillhole 204 includes a substantially vertical portion 206 coupled to aJ-section portion 208, which in turn is coupled to a substantiallyhorizontal portion 210. The J-section portion 208 as shown, has a shapethat resembles the bottom portion of the letter “J” and may be shapedsimilar to a p-trap device used in a plumbing system that is used toprevent gasses from migrating from one side of the bend to the otherside of the bend. As used in the present disclosure, “substantially” inthe context of a drillhole orientation, refers to drillholes that maynot be exactly vertical (e.g., exactly perpendicular to the terraneansurface 202) or exactly horizontal (e.g., exactly parallel to theterranean surface 202), or exactly inclined at a particular inclineangle relative to the terranean surface 202. In other words, verticaldrillholes often undulate offset from a true vertical direction, thatthey might be drilled at an angle that deviates from true vertical, andhorizontal drillholes often undulate offset from exactly horizontal.

As illustrated in this example, the three portions of the drillhole204—the vertical portion 206, the J-section portion 208, and thesubstantially horizontal portion 210—form a continuous drillhole 204that extends into the Earth. As also shown in dashed line in FIG. 2 ,the J-section portion 208 may be coupled to an inclined portion 240rather than (or in addition to) the substantially horizontal portion 210of the drillhole 204.

The illustrated drillhole 204, in this example, has a surface casing 220positioned and set around the drillhole 204 from the terranean surface202 into a particular depth in the Earth. For example, the surfacecasing 220 may be a relatively large-diameter tubular member (or stringof members) set (e.g., cemented) around the drillhole 204 in a shallowformation. As used herein, “tubular” may refer to a member that has acircular cross-section, elliptical cross-section, or other shapedcross-section. For example, in this implementation of the hazardousmaterial storage repository system 200, the surface casing 220 extendsfrom the terranean surface through a surface layer 212. The surfacelayer 212, in this example, is a geologic layer comprised of one or morelayered rock formations. In some aspects, the surface layer 212 in thisexample may or may not include freshwater aquifers, salt water or brinesources, or other sources of mobile water (e.g., water that movesthrough a geologic formation). In some aspects, the surface casing 220may isolate the drillhole 204 from such mobile water, and may alsoprovide a hanging location for other casing strings to be installed inthe drillhole 204. Further, although not shown, a conductor casing maybe set above the surface casing 220 (e.g., between the surface casing220 and the surface 202 and within the surface layer 212) to preventdrilling fluids from escaping into the surface layer 212.

As illustrated, a production casing 222 is positioned and set within thedrillhole 204 downhole of the surface casing 220. Although termed a“production” casing, in this example, the casing 222 may or may not havebeen subject to hydrocarbon production operations. Thus, the casing 222refers to and includes any form of tubular member that is set (e.g.,cemented) in the drillhole 204 downhole of the surface casing 220. Insome examples of the hazardous material storage repository system 200,the production casing 222 may begin at an end of the J-section portion208 and extend throughout the substantially horizontal portion 210. Thecasing 222 could also extend into the J-section portion 208 and into thevertical portion 206.

As shown, cement 230 is positioned (e.g., pumped) around the casings 220and 222 in an annulus between the casings 220 and 222 and the drillhole204. The cement 230, for example, may secure the casings 220 and 222(and any other casings or liners of the drillhole 204) through thesubterranean layers under the terranean surface 202. In some aspects,the cement 230 may be installed along the entire length of the casings(e.g., casings 220 and 222 and any other casings), or the cement 230could be used along certain portions of the casings if adequate for aparticular drillhole 202. The cement 230 can also provide an additionallayer of confinement for the hazardous material in canisters 226.

The drillhole 204 and associated casings 220 and 222 may be formed withvarious example dimensions and at various example depths (e.g., truevertical depth, or TVD). For instance, a conductor casing (not shown)may extend down to about 120 feet TVD, with a diameter of between about28 in. and 60 in. The surface casing 220 may extend down to about 2500feet TVD, with a diameter of between about 22 in. and 48 in. Anintermediate casing (not shown) between the surface casing 220 andproduction casing 222 may extend down to about 8000 feet TVD, with adiameter of between about 16 in. and 36 in. The production casing 222may extend inclinedly (e.g., to case the substantially horizontalportion 210 and/or the inclined portion 240) with a diameter of betweenabout 11 in. and 22 in. The foregoing dimensions are merely provided asexamples and other dimensions (e.g., diameters, TVDs, lengths) arecontemplated by the present disclosure. For example, diameters and TVDsmay depend on the particular geological composition of one or more ofthe multiple subterranean layers (212, 214, and 216), particulardrilling techniques, as well as a size, shape, or design of a hazardousmaterial canister 226 that contains hazardous material to be depositedin the hazardous material storage repository system 200. In somealternative examples, the production casing 222 (or other casing in thedrillhole 204) could be circular in cross-section, elliptical incross-section, or some other shape.

As illustrated, the vertical portion 206 of the drillhole 204 extendsthrough subterranean layers 212, 214, and 216, and, in this example,lands in a subterranean layer 219. As discussed above, the surface layer212 may or may not include mobile water. Subterranean layer 214, whichis below the surface layer 212, in this example, is a mobile water layer214. For instance, mobile water layer 214 may include one or moresources of mobile water, such as freshwater aquifers, salt water orbrine, or other source of mobile water. In this example of hazardousmaterial storage repository system 200, mobile water may be water thatmoves through a subterranean layer based on a pressure differentialacross all or a part of the subterranean layer. For example, the mobilewater layer 214 may be a permeable geologic formation in which waterfreely moves (e.g., due to pressure differences or otherwise) within thelayer 214. In some aspects, the mobile water layer 214 may be a primarysource of human-consumable water in a particular geographic area.Examples of rock formations of which the mobile water layer 214 may becomposed include porous sandstones and limestones, among otherformations.

Other illustrated layers, such as the impermeable layer 216 and thestorage layer 219, may include immobile water. Immobile water, in someaspects, is water (e.g., fresh, salt, brine), that is not fit for humanor animal consumption, or both. Immobile water, in some aspects, may bewater that, by its motion through the layers 216 or 219 (or both),cannot reach the mobile water layer 214, terranean surface 202, or both,within 10,000 years or more (such as to 1,000,000 years).

Below the mobile water layer 214, in this example implementation ofhazardous material storage repository system 200, is an impermeablelayer 216. The impermeable layer 216, in this example, may not allowmobile water to pass through. Thus, relative to the mobile water layer214, the impermeable layer 216 may have low permeability, e.g., on theorder of 0.01 millidarcy permeability. Additionally, in this example,the impermeable layer 216 may be a relatively non-ductile (i.e.,brittle) geologic formation. One measure of non-ductility isbrittleness, which is the ratio of compressive stress to tensilestrength. In some examples, the brittleness of the impermeable layer 216may be between about 20 MPa and 40 MPa.

As shown in this example, the impermeable layer 216 is shallower (e.g.,closer to the terranean surface 202) than the storage layer 219. In thisexample rock formations of which the impermeable layer 216 may becomposed include, for example, certain kinds of sandstone, mudstone,clay, and slate that exhibit permeability and brittleness properties asdescribed above. In alternative examples, the impermeable layer 216 maybe deeper (e.g., further from the terranean surface 202) than thestorage layer 219. In such alternative examples, the impermeable layer216 may be composed of an igneous rock, such as granite.

Below the impermeable layer 216 is the storage layer 219. The storagelayer 219, in this example, may be chosen as the landing for thesubstantially horizontal portion 210, which stores the hazardousmaterial, for several reasons. Relative to the impermeable layer 216 orother layers, the storage layer 219 may be thick, e.g., between about100 and 200 feet of total vertical thickness. Thickness of the storagelayer 219 may allow for easier landing and directional drilling, therebyallowing the substantially horizontal portion 210 to be readily emplacedwithin the storage layer 219 during constructions (e.g., drilling). Ifformed through an approximate horizontal center of the storage layer219, the substantially horizontal portion 210 may be surrounded by about50 to 100 feet of the geologic formation that comprises the storagelayer 219. Further, the storage layer 219 may also have only immobilewater, e.g., due to a very low permeability of the layer 219 (e.g., onthe order of milli- or nanodarcys). In addition, the storage layer 219may have sufficient ductility, such that a brittleness of the rockformation that comprises the layer 219 is between about 3 MPa and 10MPa. Examples of rock formations of which the storage layer 219 may becomposed include: shale and anhydrite. Further, in some aspects,hazardous material may be stored below the storage layer, even in apermeable formation such as sandstone or limestone, if the storage layeris of sufficient geologic properties to isolate the permeable layer fromthe mobile water layer 214.

In some examples implementations of the hazardous material storagerepository system 200, the storage layer 219 (and/or the impermeablelayer 216) is composed of shale. Shale, in some examples, may haveproperties that fit within those described above for the storage layer219. For example, shale formations may be suitable for a long-termconfinement of hazardous material (e.g., in the hazardous materialcanisters 226), and for their isolation from mobile water layer 214(e.g., aquifers) and the terranean surface 202. Shale formations may befound relatively deep in the Earth, typically 3000 feet or greater, andplaced in isolation below any fresh water aquifers. Other formations mayinclude salt or other impermeable formation layer.

Shale formations (or salt or other impermeable formation layers), forinstance, may include geologic properties that enhance the long-term(e.g., thousands of years) isolation of material. Such properties, forinstance, have been illustrated through the long term storage (e.g.,tens of millions of years) of hydrocarbon fluids (e.g., gas, liquid,mixed phase fluid) without escape of such fluids into surrounding layers(e.g., mobile water layer 214). Indeed, shale has been shown to holdnatural gas for millions of years or more, giving it a proven capabilityfor long-term storage of hazardous material. Example shale formations(e.g., Marcellus, Eagle Ford, Barnett, and otherwise) has stratificationthat contains many redundant sealing layers that have been effective inpreventing movement of water, oil, and gas for millions of years, lacksmobile water, and can be expected (e.g., based on geologicalconsiderations) to seal hazardous material (e.g., fluids or solids) forthousands of years after deposit.

In some aspects, the formation of the storage layer 219 and/or theimpermeable layer 216 may form a leakage barrier, or barrier layer tofluid leakage that may be determined, at least in part, by the evidenceof the storage capacity of the layer for hydrocarbons or other fluids(e.g., carbon dioxide) for hundreds of years, thousands of years, tensof thousands of years, hundreds of thousands of years, or even millionsof years. For example, the barrier layer of the storage layer 219 and/orimpermeable layer 216 may be defined by a time constant for leakage ofthe hazardous material of more than 10,000 years (such as between 10,000years and 1,000,000 years) based on such evidence of hydrocarbon orother fluid storage.

Shale (or salt or other impermeable layer) formations may also be at asuitable depth, e.g., between 3000 and 12,000 feet TVD. Such depths aretypically below ground water aquifer (e.g., surface layer 212 and/ormobile water layer 214). Further, the presence of soluble elements inshale, including salt, and the absence of these same elements in aquiferlayers, demonstrates a fluid isolation between shale and the aquiferlayers.

Another particular quality of shale that may advantageously lend itselfto hazardous material storage is its clay content, which, in someaspects, provides a measure of ductility greater than that found inother, impermeable rock formations (e.g., impermeable layer 216). Forexample, shale may be stratified, made up of thinly alternating layersof clays (e.g., between about 20-30% clay by volume) and other minerals.Such a composition may make shale less brittle and, thus lesssusceptible to fracturing (e.g., naturally or otherwise) as compared torock formations in the impermeable layer (e.g., dolomite or otherwise).For example, rock formations in the impermeable layer 216 may havesuitable permeability for the long term storage of hazardous material,but are too brittle and commonly are fractured. Thus, such formationsmay not have sufficient sealing qualities (as evidenced through theirgeologic properties) for the long term storage of hazardous material.

The present disclosure contemplates that there may be many other layersbetween or among the illustrated subterranean layers 212, 214, 216, and219. For example, there may be repeating patterns (e.g., vertically), ofone or more of the mobile water layer 214, impermeable layer 216, andstorage layer 219. Further, in some instances, the storage layer 219 maybe directly adjacent (e.g., vertically) the mobile water layer 214,i.e., without an intervening impermeable layer 216. In some examples,all or portions of the J-section drillhole 208 and the substantiallyhorizontal portion 210 (and/or the inclined portion 240) may be formedbelow the storage layer 219, such that the storage layer 219 (e.g.,shale or other geologic formation with characteristics as describedherein) is vertically positioned between the substantially horizontalportion 210 (and/or the inclined portion 240) and the mobile water layer214.

Although not illustrated in this particular example shown in FIG. 2 , aself-healing layer (e.g., such as the self-healing layer 132) may befound below the terranean surface 202 and between, for example, thesurface 202 and one or both of the impermeable layer 216 and the storagelayer 219. In some aspects, the self-healing layer may comprise ageologic formation that can stop or impede a flow of hazardous material(whether in liquid, solid, or gaseous form) from a storage portion ofthe drillhole 204 to or toward the terranean surface 202. For example,during formation of the drillhole 204 (e.g., drilling), all are portionsof the geologic formations of the layers 212, 214, 216, and 219, may bedamaged, thereby affecting or changing their geologic characteristics(e.g., permeability).

In certain aspects, the location of the drillhole 204 may be selected soas to be formed through the self-healing layer. For example, as shown,the drillhole 204 may be formed such that at least a portion of thevertical portion 206 of the drillhole 204 is formed to pass through theself-healing layer. In some aspects, the self-healing layer comprises ageologic formation that that does not sustain cracks for extended timedurations even after being drilled therethrough. Examples of thegeologic formation in the self-healing layer include clay or dolomite.Cracks in such rock formations tend to heal, that is, they disappearrapidly with time due to the relative ductility of the material, and theenormous pressures that occur underground from the weight of theoverlying rock on the formation in the self-healing layer. In additionto providing a “healing mechanism” for cracks that occur due to theformation of the drillhole 204 (e.g., drilling or otherwise), theself-healing layer may also provide a barrier to natural faults andother cracks that otherwise could provide a pathway for hazardousmaterial leakage (e.g., fluid or solid) from the storage region (e.g.,in the substantially horizontal portion 210) to the terranean surface202, the mobile water layer 214, or both.

As shown in this example, the substantially horizontal portion 210 ofthe drillhole 204 includes a storage area 217 in a distal part of theportion 210 into which hazardous material may be retrievably placed forlong-term storage. For example, as shown, a work string 224 (e.g.,tubing, coiled tubing, wireline, or otherwise) may be extended into thecased drillhole 204 to place one or more (three shown but there may bemore or less) hazardous material canisters 226 into long term, but insome aspects, retrievable, storage in the portion 210. For example, inthe implementation shown in FIG. 2 , the work string 224 may include adownhole tool 228 that couples to the canister 226, and with each tripinto the drillhole 204, the downhole tool 228 may deposit a particularhazardous material canister 226 in the substantially horizontal portion210.

The downhole tool 228 may couple to the canister 226 by, in someaspects, a threaded connection or other type of connection, such as alatched connection. In alternative aspects, the downhole tool 228 maycouple to the canister 226 with an interlocking latch, such thatrotation (or linear movement or electric or hydraulic switches) of thedownhole tool 228 may latch to (or unlatch from) the canister 226. Inalternative aspects, the downhole tool 224 may include one or moremagnets (e.g., rare Earth magnets, electromagnets, a combinationthereof, or otherwise) which attractingly couple to the canister 226. Insome examples, the canister 226 may also include one or more magnets(e.g., rare Earth magnets, electromagnets, a combination thereof, orotherwise) of an opposite polarity as the magnets on the downhole tool224. In some examples, the canister 226 may be made from or include aferrous or other material attractable to the magnets of the downholetool 224.

As another example, each canister 226 may be positioned within thedrillhole 204 by a drillhole tractor (e.g., on a wireline or otherwise),which may push or pull the canister into the substantially horizontalportion 210 through motorized (e.g., electric) motion. As yet anotherexample, each canister 226 may include or be mounted to rollers (e.g.,wheels), so that the downhole tool 224 may push the canister 226 intothe cased drillhole 204.

In some example implementations, the canister 226, one or more of thedrillhole casings 220 and 222, or both, may be coated with afriction-reducing coating prior to the deposit operation. For example,by applying a coating (e.g., petroleum-based product, resin, ceramic, orotherwise) to the canister 226 and/or drillhole casings, the canister226 may be more easily moved through the cased drillhole 204 into thesubstantially horizontal portion 210. In some aspects, only a portion ofthe drillhole casings may be coated. For example, in some aspects, thesubstantially vertical portion 206 may not be coated, but the J-sectionportion 208 or the substantially horizontal portion 210, or both, may becoated to facilitate easier deposit and retrieval of the canister 226.

FIG. 2 also illustrates an example of a retrieval operation of hazardousmaterial in the substantially horizontal portion 210 of the drillhole204. A retrieval operation may be the opposite of a deposit operation,such that the downhole tool 224 (e.g., a fishing tool) may be run intothe drillhole 204, coupled to the last-deposited canister 226 (e.g.,threadingly, latched, by magnet, or otherwise), and pull the canister226 to the terranean surface 202. Multiple retrieval trips may be madeby the downhole tool 224 in order to retrieve multiple canisters fromthe substantially horizontal portion 210 of the drillhole 204.

Each canister 226 may enclose hazardous material. Such hazardousmaterial, in some examples, may be biological or chemical waste or otherbiological or chemical hazardous material. In some examples, thehazardous material may include nuclear material, such as spent nuclearfuel recovered from a nuclear reactor (e.g., commercial power or testreactor) or military nuclear material. For example, a gigawatt nuclearplant may produce 30 tons of spent nuclear fuel per year. The density ofthat fuel is typically close to 10 (10 gm/cm³=10 kg/liter), so that thevolume for a year of nuclear waste is about 3 m³. Spent nuclear fuel, inthe form of nuclear fuel pellets, may be taken from the reactor and notmodified. Nuclear fuel pellet are solid, although they can contain andemit a variety of radioactive gases including tritium (13 yearhalf-life), krypton-85 (10.8 year half-life), and carbon dioxidecontaining C-14 (5730 year half-life).

In some aspects, the storage layer 219 should be able to contain anyradioactive output (e.g., gases) within the layer 219, even if suchoutput escapes the canisters 226. For example, the storage layer 219 maybe selected based on diffusion times of radioactive output through thelayer 219. For example, a minimum diffusion time of radioactive outputescaping the storage layer 219 may be set at, for example, fifty times ahalf-life for any particular component of the nuclear fuel pellets.Fifty half-lives as a minimum diffusion time would reduce an amount ofradioactive output by a factor of 1×10¹⁵. As another example, setting aminimum diffusion time to thirty half-lives would reduce an amount ofradioactive output by a factor of one billion.

For example, plutonium-239 is often considered a dangerous waste productin spent nuclear fuel because of its long half-life of 24,100 years. Forthis isotope, 50 half-lives would be 1.2 million years. Plutonium-239has low solubility in water, is not volatile, and as a solid is notcapable of diffusion through a matrix of the rock formation thatcomprises the illustrated storage layer 219 (e.g., shale or otherformation). The storage layer 219, for example comprised of shale, mayoffer the capability to have such isolation times (e.g., millions ofyears) as shown by the geological history of containing gaseoushydrocarbons (e.g., methane and otherwise) for several million years. Incontrast, in conventional nuclear material storage methods, there was adanger that some plutonium might dissolve in a layer that comprisedmobile ground water upon confinement escape.

As further shown in FIG. 2 , the storage canisters 226 may be positionedfor long term storage in the substantially horizontal portion 210,which, as shown, is coupled to the vertical portion 106 of the drillhole104 through the J-section portion 208. As illustrated, the J-sectionportion 208 includes an upwardly directed portion angled toward theterranean surface 202. In some aspects, for example when there isradioactive hazardous material stored in the canisters 226, thisinclination of the J-section portion 208 (and inclination of theinclined portion 240, if formed) may provide a further degree of safetyand containment to prevent or impede the material, even if leaked fromthe canister 226, from reaching, e.g., the mobile water layer 214, thevertical portion 206 of the drillhole 204, the terranean surface 202, ora combination thereof. For example, radionuclides of concern in thehazardous material tend to be relatively buoyant or heavy (as comparedto other components of the material). Buoyant radionuclides may be thegreatest concern for leakage, since heavy elements and molecules tend tosink, and would not diffuse upward towards the terranean surface 202.Krypton gas, and particularly krypton-85, is a buoyant radioactiveelement that is heavier than air (as are most gases) but much lighterthan water. Thus, should krypton-85 be introduced into a water bath,such gas would tend to float upward towards the terranean surface 202.Iodine, on the other hand, is denser than water, and would tend todiffuse downward if introduced into a water bath.

By including the J-section portion 208 of the drillhole 204, any suchdiffusion of radioactive material (e.g., even if leaked from a canister226 and in the presence of water or other liquid in the drillhole 204 orotherwise) would be directed angularly upward toward the substantiallyhorizontal portion 210, and more specifically, toward a distal end 221of the substantially horizontal portion 210 and away from the J-sectionportion 208 (and the vertical portion 206) of the drillhole 204. Thus,leaked hazardous material, even in a diffusible gas form, would not beoffered a path (e.g., directly) to the terranean surface 202 (or themobile water layer 214) through the vertical portion 206 of thedrillhole 210. For instance, the leaked hazardous material (especiallyin gaseous form) would be directed and gathered at the distal end 221 ofthe drillhole portion 210, or, generally, within the substantiallyhorizontal portion 210 of the drillhole 204.

Alternative methods of depositing the canisters 226 into the inclineddrillhole portion 210 may also be implemented. For instance, a fluid(e.g., liquid or gas) may be circulated through the drillhole 204 tofluidly push the canisters 226 into the inclined drillhole portion 210.In some example, each canister 226 may be fluidly pushed separately. Inalternative aspects, two or more canisters 226 may be fluidly pushed,simultaneously, through the drillhole 204 for deposit into thesubstantially horizontal portion 210. The fluid can be, in some cases,water. Other examples include a drilling mud or drilling foam. In someexamples, a gas may be used to push the canisters 226 into thedrillhole, such as air, argon, or nitrogen.

In some aspects, the choice of fluid may depend at least in part on aviscosity of the fluid. For example, a fluid may be chosen with enoughviscosity to impede the drop of the canister 226 into the substantiallyvertical portion 206. This resistance or impedance may provide a safetyfactor against a sudden drop of the canister 226. The fluid may alsoprovide lubrication to reduce a sliding friction between the canister226 and the casings 220 and 222. The canister 226 can be conveyed withina casing filled with a liquid of controlled viscosity, density, andlubricant qualities. The fluid-filled annulus between the inner diameterof the casings 220 and 222 and the outer diameter of the conveyedcanister 226 represents an opening designed to dampen any high rate ofcanister motion, providing automatic passive protection in an unlikelydecoupling of the conveyed canister 226.

In some aspects, other techniques may be employed to facilitate depositof the canister 226 into the substantially horizontal portion 210. Forexample, one or more of the installed casings (e.g., casings 220 and222) may have rails to guide the storage canister 226 into the drillhole202 while reducing friction between the casings and the canister 226.The storage canister 226 and the casings (or the rails) may be made ofmaterials that slide easily against one another. The casings may have asurface that is easily lubricated, or one that is self-lubricating whensubjected to the weight of the storage canister 226.

The fluid may also be used for retrieval of the canister 226. Forexample, in an example retrieval operation, a volume within the casings220 and 222 may be filled with a compressed gas (e.g., air, nitrogen,argon, or otherwise). As the pressure increases at an end of thesubstantially horizontal portion 210, the canisters 226 may be pushedtoward the J-section portion 208, and subsequently through thesubstantially vertical portion 206 to the terranean surface.

In some aspects, the drillhole 204 may be formed for the primary purposeof long-term storage of hazardous materials. In alternative aspects, thedrillhole 204 may have been previously formed for the primary purpose ofhydrocarbon production (e.g., oil, gas). For example, storage layer 219may be a hydrocarbon bearing formation from which hydrocarbons wereproduced into the drillhole 204 and to the terranean surface 202. Insome aspects, the storage layer 219 may have been hydraulicallyfractured prior to hydrocarbon production. Further in some aspects, theproduction casing 222 may have been perforated prior to hydraulicfracturing. In such aspects, the production casing 222 may be patched(e.g., cemented) to repair any holes made from the perforating processprior to a deposit operation of hazardous material. In addition, anycracks or openings in the cement between the casing and the drillholecan also be filled at that time.

For example, in the case of spent nuclear fuel as a hazardous material,the drillhole may be formed at a particular location, e.g., near anuclear power plant, as a new drillhole provided that the location alsoincludes an appropriate storage layer 219, such as a shale formation.Alternatively, an existing well that has already produced shale gas, orone that was abandoned as “dry,” (e.g., with sufficiently low organicsthat the gas in place is too low for commercial development), may beselected as the drillhole 204. In some aspects, prior hydraulicfracturing of the storage layer 219 through the drillhole 204 may makelittle difference in the hazardous material storage capability of thedrillhole 204. But such a prior activity may also confirm the ability ofthe storage layer 219 to store gases and other fluids for millions ofyears. If, therefore, the hazardous material or output of the hazardousmaterial (e.g., radioactive gasses or otherwise) were to escape from thecanister 226 and enter the fractured formation of the storage layer 219,such fractures may allow that material to spread relatively rapidly overa distance comparable in size to that of the fractures. In some aspects,the drillhole 202 may have been drilled for a production ofhydrocarbons, but production of such hydrocarbons had failed, e.g.,because the storage layer 219 comprised a rock formation (e.g., shale orotherwise) that was too ductile and difficult to fracture forproduction, but was advantageously ductile for the long-term storage ofhazardous material.

FIG. 3 is a schematic illustration of example implementations of anotherhazardous material storage repository system, e.g., a subterraneanlocation for the long-term (e.g., tens, hundreds, or thousands of yearsor more) but retrievable safe and secure storage of hazardous material,during a deposit or retrieval operation according to the presentdisclosure. For example, turning to FIG. 3 , this figure illustrates anexample hazardous material storage repository system 300 during adeposit (or retrieval, as described below) process, e.g., duringdeployment of one or more canisters of hazardous material in asubterranean formation. As illustrated, the hazardous material storagerepository system 300 includes a drillhole 304 formed (e.g., drilled orotherwise) from a terranean surface 302 and through multiplesubterranean layers 312, 314, and 316. Although the terranean surface302 is illustrated as a land surface, terranean surface 302 may be asub-sea or other underwater surface, such as a lake or an ocean floor orother surface under a body of water. Thus, the present disclosurecontemplates that the drillhole 304 may be formed under a body of waterfrom a drilling location on or proximate the body of water.

The illustrated drillhole 304 is a directional drillhole in this exampleof hazardous material storage repository system 300. For instance, thedrillhole 304 includes a substantially vertical portion 306 coupled to acurved portion 308, which in turn is coupled to a vertically undulatingportion 310. As used in the present disclosure, “substantially” in thecontext of a drillhole orientation, refers to drillholes that may not beexactly vertical (e.g., exactly perpendicular to the terranean surface302) or exactly horizontal (e.g., exactly parallel to the terraneansurface 302), or exactly inclined at a particular incline angle relativeto the terranean surface 302. In other words, vertical drillholes oftenundulate offset from a true vertical direction, that they might bedrilled at an angle that deviates from true vertical, and horizontaldrillholes often undulate offset from exactly horizontal. Further, insome aspects, an undulating portion may not undulate with regularity,i.e., have peaks that are uniformly spaced apart or valleys that areuniformly spaced apart. Instead, an undulating drillhole may undulateirregularly, e.g., with peaks that are non-uniformly spaced and/orvalleys that are non-uniformly spaced. Further, an undulated drillholemay have a peak-to-valley distance that varies along a length of thedrillhole. As illustrated in this example, the three portions of thedrillhole 304—the vertical portion 306, the curved portion 308, and thevertically undulating portion 310—form a continuous drillhole 304 thatextends into the Earth.

The illustrated drillhole 304, in this example, has a surface casing 320positioned and set around the drillhole 304 from the terranean surface302 into a particular depth in the Earth. For example, the surfacecasing 320 may be a relatively large-diameter tubular member (or stringof members) set (e.g., cemented) around the drillhole 304 in a shallowformation. As used herein, “tubular” may refer to a member that has acircular cross-section, elliptical cross-section, or other shapedcross-section. For example, in this implementation of the hazardousmaterial storage repository system 300, the surface casing 320 extendsfrom the terranean surface through a surface layer 312. The surfacelayer 312, in this example, is a geologic layer comprised of one or morelayered rock formations. In some aspects, the surface layer 312 in thisexample may or may not include freshwater aquifers, salt water or brinesources, or other sources of mobile water (e.g., water that movesthrough a geologic formation). In some aspects, the surface casing 320may isolate the drillhole 304 from such mobile water, and may alsoprovide a hanging location for other casing strings to be installed inthe drillhole 304. Further, although not shown, a conductor casing maybe set above the surface casing 320 (e.g., between the surface casing320 and the surface 302 and within the surface layer 312) to preventdrilling fluids from escaping into the surface layer 312.

As illustrated, a production casing 322 is positioned and set within thedrillhole 304 downhole of the surface casing 320. Although termed a“production” casing, in this example, the casing 322 may or may not havebeen subject to hydrocarbon production operations. Thus, the casing 322refers to and includes any form of tubular member that is set (e.g.,cemented) in the drillhole 304 downhole of the surface casing 320. Insome examples of the hazardous material storage repository system 300,the production casing 322 may begin at an end of the curved portion 308and extend throughout the vertically undulating portion 310. The casing322 could also extend into the curved portion 308 and into the verticalportion 306.

As shown, cement 330 is positioned (e.g., pumped) around the casings 320and 322 in an annulus between the casings 320 and 322 and the drillhole304. The cement 330, for example, may secure the casings 320 and 322(and any other casings or liners of the drillhole 304) through thesubterranean layers under the terranean surface 302. In some aspects,the cement 330 may be installed along the entire length of the casings(e.g., casings 320 and 322 and any other casings), or the cement 330could be used along certain portions of the casings if adequate for aparticular drillhole 302. The cement 330 can also provide an additionallayer of confinement for the hazardous material in canisters 326.

The drillhole 304 and associated casings 320 and 322 may be formed withvarious example dimensions and at various example depths (e.g., truevertical depth, or TVD). For instance, a conductor casing (not shown)may extend down to about 120 feet TVD, with a diameter of between about28 in. and 60 in. The surface casing 320 may extend down to about 2500feet TVD, with a diameter of between about 22 in. and 48 in. Anintermediate casing (not shown) between the surface casing 320 andproduction casing 322 may extend down to about 8000 feet TVD, with adiameter of between about 16 in. and 36 in. The production casing 322may extend inclinedly (e.g., to case the vertically undulating portion310) with a diameter of between about 11 in. and 22 in. The foregoingdimensions are merely provided as examples and other dimensions (e.g.,diameters, TVDs, lengths) are contemplated by the present disclosure.For example, diameters and TVDs may depend on the particular geologicalcomposition of one or more of the multiple subterranean layers (312,314, and 316), particular drilling techniques, as well as a size, shape,or design of a hazardous material canister 326 that contains hazardousmaterial to be deposited in the hazardous material storage repositorysystem 300. In some alternative examples, the production casing 322 (orother casing in the drillhole 304) could be circular in cross-section,elliptical in cross-section, or some other shape.

As illustrated, the vertical portion 306 of the drillhole 304 extendsthrough subterranean layers 312, 314, and 316, and, in this example,lands in a subterranean layer 319. As discussed above, the surface layer312 may or may not include mobile water. Subterranean layer 314, whichis below the surface layer 312, in this example, is a mobile water layer314. For instance, mobile water layer 314 may include one or moresources of mobile water, such as freshwater aquifers, salt water orbrine, or other source of mobile water. In this example of hazardousmaterial storage repository system 300, mobile water may be water thatmoves through a subterranean layer based on a pressure differentialacross all or a part of the subterranean layer. For example, the mobilewater layer 314 may be a permeable geologic formation in which waterfreely moves (e.g., due to pressure differences or otherwise) within thelayer 314. In some aspects, the mobile water layer 314 may be a primarysource of human-consumable water in a particular geographic area.Examples of rock formations of which the mobile water layer 314 may becomposed include porous sandstones and limestones, among otherformations.

Other illustrated layers, such as the impermeable layer 316 and thestorage layer 319, may include immobile water. Immobile water, in someaspects, is water (e.g., fresh, salt, brine), that is not fit for humanor animal consumption, or both. Immobile water, in some aspects, may bewater that, by its motion through the layers 316 or 319 (or both),cannot reach the mobile water layer 314, terranean surface 302, or both,within 10,000 years or more (such as to 1,000,000 years).

Below the mobile water layer 314, in this example implementation ofhazardous material storage repository system 300, is an impermeablelayer 316. The impermeable layer 316, in this example, may not allowmobile water to pass through. Thus, relative to the mobile water layer314, the impermeable layer 316 may have low permeability, e.g., on theorder of nanodarcy permeability. Additionally, in this example, theimpermeable layer 316 may be a relatively non-ductile (i.e., brittle)geologic formation. One measure of non-ductility is brittleness, whichis the ratio of compressive stress to tensile strength. In someexamples, the brittleness of the impermeable layer 316 may be betweenabout 20 MPa and 40 MPa.

As shown in this example, the impermeable layer 316 is shallower (e.g.,closer to the terranean surface 302) than the storage layer 319. In thisexample rock formations of which the impermeable layer 316 may becomposed include, for example, certain kinds of sandstone, mudstone,clay, and slate that exhibit permeability and brittleness properties asdescribed above. In alternative examples, the impermeable layer 316 maybe deeper (e.g., further from the terranean surface 302) than thestorage layer 319. In such alternative examples, the impermeable layer316 may be composed of an igneous rock, such as granite.

Below the impermeable layer 316 is the storage layer 319. The storagelayer 319, in this example, may be chosen as the landing for thevertically undulating portion 310, which stores the hazardous material,for several reasons. Relative to the impermeable layer 316 or otherlayers, the storage layer 319 may be thick, e.g., between about 100 and200 feet of total vertical thickness. Thickness of the storage layer 319may allow for easier landing and directional drilling, thereby allowingthe vertically undulating portion 310 to be readily emplaced within thestorage layer 319 during constructions (e.g., drilling). If formedthrough an approximate horizontal center of the storage layer 319, thevertically undulating portion 310 may be surrounded by about 50 to 100feet of the geologic formation that comprises the storage layer 319.Further, the storage layer 319 may also have only immobile water, e.g.,due to a very low permeability of the layer 319 (e.g., on the order ofmilli- or nanodarcys). In addition, the storage layer 319 may havesufficient ductility, such that a brittleness of the rock formation thatcomprises the layer 319 is between about 3 MPa and 10 MPa. Examples ofrock formations of which the storage layer 319 may be composed include:shale and anhydrite. Further, in some aspects, hazardous material may bestored below the storage layer, even in a permeable formation such assandstone or limestone, if the storage layer is of sufficient geologicproperties to isolate the permeable layer from the mobile water layer314.

In some examples implementations of the hazardous material storagerepository system 300, the storage layer 319 (and/or the impermeablelayer 316) is composed of shale. Shale, in some examples, may haveproperties that fit within those described above for the storage layer319. For example, shale formations may be suitable for a long-termconfinement of hazardous material (e.g., in the hazardous materialcanisters 326), and for their isolation from mobile water layer 314(e.g., aquifers) and the terranean surface 302. Shale formations may befound relatively deep in the Earth, typically 3000 feet or greater, andplaced in isolation below any fresh water aquifers. Other formations mayinclude salt or other impermeable formation layer.

Shale formations (or salt or other impermeable formation layers), forinstance, may include geologic properties that enhance the long-term(e.g., thousands of years) isolation of material. Such properties, forinstance, have been illustrated through the long term storage (e.g.,tens of millions of years) of hydrocarbon fluids (e.g., gas, liquid,mixed phase fluid) without escape of such fluids into surrounding layers(e.g., mobile water layer 314). Indeed, shale has been shown to holdnatural gas for millions of years or more, giving it a proven capabilityfor long-term storage of hazardous material. Example shale formations(e.g., Marcellus, Eagle Ford, Barnett, and otherwise) has stratificationthat contains many redundant sealing layers that have been effective inpreventing movement of water, oil, and gas for millions of years, lacksmobile water, and can be expected (e.g., based on geologicalconsiderations) to seal hazardous material (e.g., fluids or solids) forthousands of years after deposit.

In some aspects, the formation of the storage layer 319 and/or theimpermeable layer 316 may form a leakage barrier, or barrier layer tofluid leakage that may be determined, at least in part, by the evidenceof the storage capacity of the layer for hydrocarbons or other fluids(e.g., carbon dioxide) for hundreds of years, thousands of years, tensof thousands of years, hundreds of thousands of years, or even millionsof years. For example, the barrier layer of the storage layer 319 and/orimpermeable layer 316 may be defined by a time constant for leakage ofthe hazardous material more than 10,000 years (such as between 10,000years and 1,000,000 years) based on such evidence of hydrocarbon orother fluid storage.

Shale (or salt or other impermeable layer) formations may also be at asuitable depth, e.g., between 3000 and 12,000 feet TVD. Such depths aretypically below ground water aquifer (e.g., surface layer 312 and/ormobile water layer 314). Further, the presence of soluble elements inshale, including salt, and the absence of these same elements in aquiferlayers, demonstrates a fluid isolation between shale and the aquiferlayers.

Another particular quality of shale that may advantageously lend itselfto hazardous material storage is its clay content, which, in someaspects, provides a measure of ductility greater than that found inother, impermeable rock formations (e.g., impermeable layer 316). Forexample, shale may be stratified, made up of thinly alternating layersof clays (e.g., between about 20-30% clay by volume) and other minerals.Such a composition may make shale less brittle and, thus lesssusceptible to fracturing (e.g., naturally or otherwise) as compared torock formations in the impermeable layer (e.g., dolomite or otherwise).For example, rock formations in the impermeable layer 316 may havesuitable permeability for the long term storage of hazardous material,but are too brittle and commonly are fractured. Thus, such formationsmay not have sufficient sealing qualities (as evidenced through theirgeologic properties) for the long term storage of hazardous material.

The present disclosure contemplates that there may be many other layersbetween or among the illustrated subterranean layers 312, 314, 316, and319. For example, there may be repeating patterns (e.g., vertically), ofone or more of the mobile water layer 314, impermeable layer 316, andstorage layer 319. Further, in some instances, the storage layer 319 maybe directly adjacent (e.g., vertically) the mobile water layer 314,i.e., without an intervening impermeable layer 316. In some examples,all or portions of the curved portion 308 and the vertically undulatingportion 310 may be formed below the storage layer 319, such that thestorage layer 319 (e.g., shale or other geologic formation withcharacteristics as described herein) is vertically positioned betweenthe vertically undulating portion 310 and the mobile water layer 314.

Although not illustrated in this particular example shown in FIG. 3 , aself-healing layer (e.g., such as the self-healing layer 132) may befound below the terranean surface 302 and between, for example, thesurface 302 and one or both of the impermeable layer 316 and the storagelayer 319. In some aspects, the self-healing layer may comprise ageologic formation that can stop or impede a flow of hazardous material(whether in liquid, solid, or gaseous form) from a storage portion ofthe drillhole 304 to or toward the terranean surface 302. For example,during formation of the drillhole 304 (e.g., drilling), all are portionsof the geologic formations of the layers 312, 314, 316, and 319, may bedamaged, thereby affecting or changing their geologic characteristics(e.g., permeability).

In certain aspects, the location of the drillhole 304 may be selected soas to be formed through the self-healing layer. For example, as shown,the drillhole 304 may be formed such that at least a portion of thevertical portion 306 of the drillhole 304 is formed to pass through theself-healing layer. In some aspects, the self-healing layer comprises ageologic formation that that does not sustain cracks for extended timedurations even after being drilled therethrough. Examples of thegeologic formation in the self-healing layer include clay or dolomite.Cracks in such rock formations tend to heal, that is, they disappearrapidly with time due to the relative ductility of the material, and theenormous pressures that occur underground from the weight of theoverlying rock on the formation in the self-healing layer. In additionto providing a “healing mechanism” for cracks that occur due to theformation of the drillhole 304 (e.g., drilling or otherwise), theself-healing layer may also provide a barrier to natural faults andother cracks that otherwise could provide a pathway for hazardousmaterial leakage (e.g., fluid or solid) from the storage region (e.g.,in the vertically undulating portion 310) to the terranean surface 302,the mobile water layer 314, or both.

As shown in this example, the vertically undulating portion 310 of thedrillhole 304 includes a storage area 317 in a distal part of theportion 310 into which hazardous material may be retrievably placed forlong-term storage. For example, as shown, a work string 324 (e.g.,tubing, coiled tubing, wireline, or otherwise) may be extended into thecased drillhole 304 to place one or more (three shown but there may bemore or less) hazardous material canisters 326 into long term, but insome aspects, retrievable, storage in the portion 310. For example, inthe implementation shown in FIG. 3 , the work string 324 may include adownhole tool 328 that couples to the canister 326, and with each tripinto the drillhole 304, the downhole tool 328 may deposit a particularhazardous material canister 326 in the vertically undulating portion310.

The downhole tool 328 may couple to the canister 326 by, in someaspects, a threaded connection or other type of connection, such as alatched connection. In alternative aspects, the downhole tool 328 maycouple to the canister 326 with an interlocking latch, such thatrotation (or linear movement or electric or hydraulic switches) of thedownhole tool 328 may latch to (or unlatch from) the canister 326. Inalternative aspects, the downhole tool 324 may include one or moremagnets (e.g., rare Earth magnets, electromagnets, a combinationthereof, or otherwise) which attractingly couple to the canister 326. Insome examples, the canister 326 may also include one or more magnets(e.g., rare Earth magnets, electromagnets, a combination thereof, orotherwise) of an opposite polarity as the magnets on the downhole tool324. In some examples, the canister 326 may be made from or include aferrous or other material attractable to the magnets of the downholetool 324.

As another example, each canister 326 may be positioned within thedrillhole 304 by a drillhole tractor (e.g., on a wireline or otherwise),which may push or pull the canister into the vertically undulatingportion 310 through motorized (e.g., electric) motion. As yet anotherexample, each canister 326 may include or be mounted to rollers (e.g.,wheels), so that the downhole tool 324 may push the canister 326 intothe cased drillhole 304.

In some example implementations, the canister 326, one or more of thedrillhole casings 320 and 322, or both, may be coated with afriction-reducing coating prior to the deposit operation. For example,by applying a coating (e.g., petroleum-based product, resin, ceramic, orotherwise) to the canister 326 and/or drillhole casings, the canister326 may be more easily moved through the cased drillhole 304 into thevertically undulating portion 310. In some aspects, only a portion ofthe drillhole casings may be coated. For example, in some aspects, thesubstantially vertical portion 306 may not be coated, but the curvedportion 308 or the vertically undulating portion 310, or both, may becoated to facilitate easier deposit and retrieval of the canister 326.

FIG. 3 also illustrates an example of a retrieval operation of hazardousmaterial in the vertically undulating portion 310 of the drillhole 304.A retrieval operation may be the opposite of a deposit operation, suchthat the downhole tool 324 (e.g., a fishing tool) may be run into thedrillhole 304, coupled to the last-deposited canister 326 (e.g.,threadingly, latched, by magnet, or otherwise), and pull the canister326 to the terranean surface 302. Multiple retrieval trips may be madeby the downhole tool 324 in order to retrieve multiple canisters fromthe vertically undulating portion 310 of the drillhole 304.

Each canister 326 may enclose hazardous material. Such hazardousmaterial, in some examples, may be biological or chemical waste or otherbiological or chemical hazardous material. In some examples, thehazardous material may include nuclear material, such as spent nuclearfuel recovered from a nuclear reactor (e.g., commercial power or testreactor) or military nuclear material. For example, a gigawatt nuclearplant may produce 30 tons of spent nuclear fuel per year. The density ofthat fuel is typically close to 10 (10 gm/cm³=10 kg/liter), so that thevolume for a year of nuclear waste is about 3 m³. Spent nuclear fuel, inthe form of nuclear fuel pellets, may be taken from the reactor and notmodified. Nuclear fuel pellet are solid, although they can contain andemit a variety of radioactive gases including tritium (13 yearhalf-life), krypton-85 (10.8 year half-life), and carbon dioxidecontaining C-14 (5730 year half-life).

In some aspects, the storage layer 319 should be able to contain anyradioactive output (e.g., gases) within the layer 319, even if suchoutput escapes the canisters 326. For example, the storage layer 319 maybe selected based on diffusion times of radioactive output through thelayer 319. For example, a minimum diffusion time of radioactive outputescaping the storage layer 319 may be set at, for example, fifty times ahalf-life for any particular component of the nuclear fuel pellets.Fifty half-lives as a minimum diffusion time would reduce an amount ofradioactive output by a factor of 1×10¹⁵. As another example, setting aminimum diffusion time to thirty half-lives would reduce an amount ofradioactive output by a factor of one billion.

For example, plutonium-239 is often considered a dangerous waste productin spent nuclear fuel because of its long half-life of 24,100 years. Forthis isotope, 50 half-lives would be 1.2 million years. Plutonium-239has low solubility in water, is not volatile, and as a solid is notcapable of diffusion through a matrix of the rock formation thatcomprises the illustrated storage layer 319 (e.g., shale or otherformation). The storage layer 319, for example comprised of shale, mayoffer the capability to have such isolation times (e.g., millions ofyears) as shown by the geological history of containing gaseoushydrocarbons (e.g., methane and otherwise) for several million years. Incontrast, in conventional nuclear material storage methods, there was adanger that some plutonium might dissolve in a layer that comprisedmobile ground water upon confinement escape.

As further shown in FIG. 3 , the storage canisters 326 may be positionedfor long term storage in the vertically undulating portion 310, which,as shown, is coupled to the vertical portion 106 of the drillhole 104through the curved portion 308. As illustrated, the curved portion 308includes an upwardly directed portion angled toward the terraneansurface 302. Further, as shown, the undulating portion 310 of thedrillhole 304 includes several upwardly and downwardly (relative to thesurface 302) inclined portions, thereby forming several peaks andvalleys in the undulating portion 310. In some aspects, for example whenthere is radioactive hazardous material stored in the canisters 326,these inclinations of the curved portion 308 and undulating portion 310may provide a further degree of safety and containment to prevent orimpede the material, even if leaked from the canister 326, fromreaching, e.g., the mobile water layer 314, the vertical portion 306 ofthe drillhole 304, the terranean surface 302, or a combination thereof.For example, radionuclides of concern in the hazardous material tend tobe relatively buoyant or heavy (as compared to other components of thematerial). Buoyant radionuclides may be the greatest concern forleakage, since heavy elements and molecules tend to sink, and would notdiffuse upward towards the terranean surface 302. Krypton gas, andparticularly krypton-85, is a buoyant radioactive element that isheavier than air (as are most gases) but much lighter than water. Thus,should krypton-85 be introduced into a water bath, such gas would tendto float upward towards the terranean surface 302. Iodine, on the otherhand, is denser than water, and would tend to diffuse downward ifintroduced into a water bath.

By including the curved portion 308 of the drillhole 304 and theundulating portion 310, any such diffusion of radioactive material(e.g., even if leaked from a canister 326 and in the presence of wateror other liquid in the drillhole 304 or otherwise) would be directedtoward the vertically undulating portion 310, and more specifically, topeaks within the vertically undulating portion 310 and away from thecurved portion 308 (and the vertical portion 306) of the drillhole 304.Thus, leaked hazardous material, even in a diffusible gas form, wouldnot be offered a path (e.g., directly) to the terranean surface 302 (orthe mobile water layer 314) through the vertical portion 306 of thedrillhole 310. For instance, the leaked hazardous material (especiallyin gaseous form) would be directed and gathered at the peaks of thedrillhole portion 310, or, generally, within the vertically undulatingportion 310 of the drillhole 304.

Alternative methods of depositing the canisters 326 into the inclineddrillhole portion 310 may also be implemented. For instance, a fluid(e.g., liquid or gas) may be circulated through the drillhole 304 tofluidly push the canisters 326 into the inclined drillhole portion 310.In some example, each canister 326 may be fluidly pushed separately. Inalternative aspects, two or more canisters 326 may be fluidly pushed,simultaneously, through the drillhole 304 for deposit into thevertically undulating portion 310. The fluid can be, in some cases,water. Other examples include a drilling mud or drilling foam. In someexamples, a gas may be used to push the canisters 326 into thedrillhole, such as air, argon, or nitrogen.

In some aspects, the choice of fluid may depend at least in part on aviscosity of the fluid. For example, a fluid may be chosen with enoughviscosity to impede the drop of the canister 326 into the substantiallyvertical portion 306. This resistance or impedance may provide a safetyfactor against a sudden drop of the canister 326. The fluid may alsoprovide lubrication to reduce a sliding friction between the canister326 and the casings 320 and 322. The canister 326 can be conveyed withina casing filled with a liquid of controlled viscosity, density, andlubricant qualities. The fluid-filled annulus between the inner diameterof the casings 320 and 322 and the outer diameter of the conveyedcanister 326 represents an opening designed to dampen any high rate ofcanister motion, providing automatic passive protection in an unlikelydecoupling of the conveyed canister 326.

In some aspects, other techniques may be employed to facilitate depositof the canister 326 into the vertically undulating portion 310. Forexample, one or more of the installed casings (e.g., casings 320 and322) may have rails to guide the storage canister 326 into the drillhole302 while reducing friction between the casings and the canister 326.The storage canister 326 and the casings (or the rails) may be made ofmaterials that slide easily against one another. The casings may have asurface that is easily lubricated, or one that is self-lubricating whensubjected to the weight of the storage canister 326.

The fluid may also be used for retrieval of the canister 326. Forexample, in an example retrieval operation, a volume within the casings320 and 322 may be filled with a compressed gas (e.g., air, nitrogen,argon, or otherwise). As the pressure increases at an end of thevertically undulating portion 310, the canisters 326 may be pushedtoward the curved portion 308, and subsequently through thesubstantially vertical portion 306 to the terranean surface.

In some aspects, the drillhole 304 may be formed for the primary purposeof long-term storage of hazardous materials. In alternative aspects, thedrillhole 304 may have been previously formed for the primary purpose ofhydrocarbon production (e.g., oil, gas). For example, storage layer 319may be a hydrocarbon bearing formation from which hydrocarbons wereproduced into the drillhole 304 and to the terranean surface 302. Insome aspects, the storage layer 319 may have been hydraulicallyfractured prior to hydrocarbon production. Further in some aspects, theproduction casing 322 may have been perforated prior to hydraulicfracturing. In such aspects, the production casing 322 may be patched(e.g., cemented) to repair any holes made from the perforating processprior to a deposit operation of hazardous material. In addition, anycracks or openings in the cement between the casing and the drillholecan also be filled at that time.

For example, in the case of spent nuclear fuel as a hazardous material,the drillhole may be formed at a particular location, e.g., near anuclear power plant, as a new drillhole provided that the location alsoincludes an appropriate storage layer 319, such as a shale formation.Alternatively, an existing well that has already produced shale gas, orone that was abandoned as “dry,” (e.g., with sufficiently low organicsthat the gas in place is too low for commercial development), may beselected as the drillhole 304. In some aspects, prior hydraulicfracturing of the storage layer 319 through the drillhole 304 may makelittle difference in the hazardous material storage capability of thedrillhole 304. But such a prior activity may also confirm the ability ofthe storage layer 319 to store gases and other fluids for millions ofyears. If, therefore, the hazardous material or output of the hazardousmaterial (e.g., radioactive gasses or otherwise) were to escape from thecanister 326 and enter the fractured formation of the storage layer 319,such fractures may allow that material to spread relatively rapidly overa distance comparable in size to that of the fractures. In some aspects,the drillhole 302 may have been drilled for a production ofhydrocarbons, but production of such hydrocarbons had failed, e.g.,because the storage layer 319 comprised a rock formation (e.g., shale orotherwise) that was too ductile and difficult to fracture forproduction, but was advantageously ductile for the long-term storage ofhazardous material.

FIG. 4A-4C are schematic illustrations of other example implementationsof a hazardous material storage repository system according to thepresent disclosure. FIG. 4A shows hazardous material storage repositorysystem 400, FIG. 4B shows hazardous material storage repository system450, and FIG. 4C shows hazardous material storage repository system 480.Each of the systems 400, 450, and 480 include a substantially verticaldrillhole (404, 454, and 484, respectively) drilled from a terraneansurface (402, 452, and 482, respectively). Each substantially verticaldrillhole (404, 454, 484) couples to (or continues into) a transitiondrillhole (406, 456, and 486, respectively) that is a curved orradiussed drillhole. Each transition drillhole (406, 456, and 486) thencouples to (or continues into) an isolation drillhole (408, 458, and488, respectively) that includes or comprises a hazardous materialstorage repository into which one or more hazardous material storagecanisters (e.g., canisters 126) may be placed for long-term storage and,if necessary retrieved according to the present disclosure.

As shown in FIG. 4A, the isolation drillhole 408 is a spiral drillholethat, at the point where it connects to the transition drillhole 406,starts to curve to the horizontal and simultaneously begins to curve tothe side, i.e. in a horizontal direction. Once the spiral drillholereaches its lowest point, it continues to curve in both directions,giving it a slight upward spiral. At that point the horizontal curve maybe made a little bigger so that the curve does not intersect thevertical drillhole 404. Once the spiral drillhole begins to rise, acurved hazardous material storage repository section may commence. Thestorage section may continue until a highest point (e.g., point closestto the terranean surface 402), which is a dead-end trap (e.g., forescaped hazardous material solid, liquid, or gas). The rise of thespiral drillhole can be typically 3 degrees.

In some aspects, the path of the spiral drillhole 408 can be down theaxis of the spiral (that is, in the center of the spiraling circles) ordisplaced. Also, as shown in FIG. 4A, the vertical drillhole 404 isformed within the spiral drillhole 408. In other words, the spiraldrillhole 408 may be formed symmetrically around the vertical drillhole404. Turning briefly to FIG. 4C, the system 480 shows a spiral drillhole488 similar to that of the spiral drillhole 408. However, spiraldrillhole 488 is formed offset and to a side of the vertical drillhole484. In some aspects, the spiral drillhole 488 can be formed offset ofany side of the vertical drillhole 484.

Turning to FIG. 4B, the system 450 includes a spiral drillhole 458 thatis coupled to the transition drillhole 456 that turns from the verticaldrillhole 454. Here, the spiral drillhole 458, rather than beingoriented vertically (e.g., with an axis of rotation parallel of thevertical drillhole), is oriented horizontally (e.g., with an axis ofrotation perpendicular to the vertical drillhole 454). At an end of orwithin the spiral drillhole 458 (or both) is a hazardous materialstorage section.

In the implementations of systems 400, 450, and 480, a radius ofcurvature of the transition drillholes may be about 1000 feet. Thecircumference of each spiral in the spiral drillholes may be about atimes the radius of curvature, or about 6,000 feet. Thus, each spiral inthe spiral drillholes may contain a bit over one mile of storage area ofhazardous material canisters. In some alternative aspects, the radius ofcurvature may be about 500 feet. Then, each spiral of the spiraldrillholes may include about 0.5 miles of storage area of hazardousmaterial canisters. If two miles of storage is desired then there may befour spirals for each spiral drillholes of this size.

As shown in FIGS. 4A-4C, each of the systems 400, 450, and 480 includedrillhole portions that serve as hazardous material storage areas andare directed vertically toward the terranean surface and away from anintersection between the transition drillhole of each system and thevertical drillhole of each section. Thus, any leaked hazardous material(e.g., such as radioactive waste gas) may be directed to suchvertically-directed storage areas and away from the vertical drillholes.Each of the drillholes shown in FIGS. 4A-4C may be cased or uncased; thecasing may serve as an additional layer of protection to preventhazardous material from reaching mobile water. If casing is omitted,then angular changes to any drillhole can be more rapid with aconstraint being the accommodation of movement of any canistertherethrough. If there is casing, angular changes in direction for thedrillholes may be done sufficiently slowly (as they are in standarddirectional drilling) that the casing can be pushed into the drillhole.Further, in some aspects, all or a portion of each of the illustratedisolation drillholes (408, 458, and 488) may be formed in or under animpermeable layer (as described in the present disclosure).

In some aspects, implementations of a spiral drillhole may have aconstant curvature around an axis of rotation. Alternativeimplementations of a spiral drillhole may have a gradually changingcurvature, making the spirals in the spiral drillhole either tighter orless confined. Still additional implementations of a spiral drillholemay have the spirals changing in radius (making it tighter or lesstight) but have little or no vertical rise (e.g., for situations inwhich it might be useful if the geologic layer in which the hazardousmaterial storage section of the isolation drillholes is not very thickin the vertical dimension).

FIG. 5A is a top view, and FIGS. 5B-5C are side views, of schematicillustrations of another example implementation of a hazardous materialstorage repository system 500. As shown, the system includes a verticaldrillhole 504 formed from a terranean surface 502. The verticaldrillhole 504 is coupled to or continues into a transition drillhole506. The transition drillhole 506 is coupled to or turns into anisolation drillhole 508. In this example, the isolation drillhole 508includes or comprise an undulating drillhole in which the undulationsare substantially side-to-side. As shown in FIG. 5B, the isolationdrillhole 508 rises toward the terranean surface 502 and vertically awayfrom the transition drillhole 506 as it undulates side-to-side. As shownin FIG. 5C, alternatively, the isolation drillhole 508 stays in a planesubstantially parallel to the terranean surface 502 as it undulatesside-to-side.

In some aspects, the spiral or undulating drillholes may be orientedwithout regard to the stress pattern of any gas or oil bearing layer inwhich they are formed. This is because the orientation need not takeinto account any fracturing of the drillhole as is the case forhydrocarbon production. Thus, drillhole geometers that are not orientedin the direction of the rock stress pattern, and are more compact, canbe utilized. These drillholes may also have significant value inreducing the amount of terranean land under which the drillholes areformed. This may also reduce a cost of the land and of any mineralrights that must be bought to allow the hazardous material storagerepository systems to be built. The drillholes are therefore determinednot by the pattern of stresses in the rock, but primarily by theefficient and practical use of the available land.

Each of the drillholes shown in FIGS. 5A-5C may be cased or uncased; thecasing may serve as an additional layer of protection to preventhazardous material from reaching mobile water. If casing is omitted,then angular changes to any drillhole can be more rapid with aconstraint being the accommodation of movement of any canistertherethrough. If there is casing, angular changes in direction for thedrillholes may be done sufficiently slowly (as they are in standarddirectional drilling) that the casing can be pushed into the drillhole.Further, in some aspects, all or a portion of the isolation drillhole508 may be formed in or under an impermeable layer (as described in thepresent disclosure).

Referring generally to FIGS. 1A, 2, 3, 4A-4C, and 5A-5C, the examplehazardous material storage repository systems (e.g., 100, 200, 300, 400,450, 480, and 500) may provide for multiple layers of containment toensure that a hazardous material (e.g., biological, chemical, nuclear)is sealingly stored in an appropriate subterranean layer. In someexample implementations, there may be at least twelve layers ofcontainment. In alternative implementations, a fewer or a greater numberof containment layers may be employed.

First, using spent nuclear fuel as an example hazardous material, thefuel pellets are taken from the reactor and not modified. They may bemade from sintered uranium dioxide (UO2), a ceramic, and may remainsolid and emit a variety of radioactive gases including tritium (13 yearhalf-life), krypton-85 (10.8 year half-life), and carbon dioxidecontaining C-14 (5730 year half-life). Unless the pellets are exposed toextremely corrosive conditions or other effects that damage the multiplelayers of containment, most of the radioisotopes (including the C-14,tritium or krypton-85) will be contained in the pellets.

Second, the fuel pellets are surrounded by the zircaloy tubes of thefuel rods, just as in the reactor. As described, the tubes could bemounted in the original fuel assemblies, or removed from thoseassemblies for tighter packing.

Third, the tubes are placed in the sealed housings of the hazardousmaterial canister. The housing may be a unified structure or multi-panelstructure, with the multiple panels (e.g., sides, top, bottom)mechanically fastened (e.g., screws, rivets, welds, and otherwise).

Fourth, a material (e.g., solid or fluid) may fill the hazardousmaterial canister to provide a further buffer between the material andthe exterior of the canister.

Fifth, the hazardous material canister(s) are positioned (as describedabove), in a drillhole that is lined with a steel or other sealingcasing that extends, in some examples, throughout the entire drillhole(e.g., a substantially vertical portion, a radiussed portion, and ainclined portion). The casing is cemented in place, providing arelatively smooth surface (e.g., as compared to the drillhole wall) forthe hazardous material canister to be moved through, thereby reducingthe possibility of a leak or break during deposit or retrieval.

Sixth, the cement that holds or helps hold the casing in place, may alsoprovide a sealing layer to contain the hazardous material should itescape the canister.

Seventh, the hazardous material canister is stored in a portion of thedrillhole (e.g., the inclined portion) that is positioned within a thick(e.g., 100-200 feet) seam of a rock formation that comprises a storagelayer. The storage layer may be chosen due at least in part to thegeologic properties of the rock formation (e.g., only immobile water,low permeability, thick, appropriate ductility or non-brittleness). Forexample, in the case of shale as the rock formation of the storagelayer, this type of rock may offers a level of containment since it isknown that shale has been a seal for hydrocarbon gas for millions ofyears. The shale may contain brine, but that brine is demonstrablyimmobile, and not in communication with surface fresh water.

Eighth, in some aspects, the rock formation of the storage layer mayhave other unique geological properties that offer another level ofcontainment. For example, shale rock often contains reactive components,such as iron sulfide, that reduce the likelihood that hazardousmaterials (e.g., spent nuclear fuel and its radioactive output) canmigrate through the storage layer without reacting in ways that reducethe diffusion rate of such output even further. Further, the storagelayer may include components, such as clay and organic matter, thattypically have extremely low diffusivity. For example, shale may bestratified and composed of thinly alternating layers of clays and otherminerals. Such a stratification of a rock formation in the storagelayer, such as shale, may offer this additional layer of containment.

Ninth, the storage layer may be located deeper than, and under, animpermeable layer, which separates the storage layer (e.g., vertically)from a mobile water layer.

Tenth, the storage layer may be selected based on a depth (e.g., 3000 to12,000 ft.) of such a layer within the subterranean layers. Such depthsare typically far below any layers that contain mobile water, and thus,the sheer depth of the storage layer provides an additional layer ofcontainment.

Eleventh, example implementations of the hazardous material storagerepository system of the present disclosure facilitate monitoring of thestored hazardous material. For example, if monitored data indicates aleak or otherwise of the hazardous material (e.g., change intemperature, radioactivity, or otherwise), or even tampering orintrusion of the canister, the hazardous material canister may beretrieved for repair or inspection.

Twelfth, the one or more hazardous material canisters may be retrievablefor periodic inspection, conditioning, or repair, as necessary (e.g.,with or without monitoring). Thus, any problem with the canisters may beaddressed without allowing hazardous material to leak or escape from thecanisters unabated.

Thirteenth, even if hazardous material escaped from the canisters and noimpermeable layer was located between the leaked hazardous material andthe terranean surface, the leaked hazardous material may be containedwithin the drillhole at a location that has no upward path to thesurface or to aquifers (e.g., mobile water layers) or to other zonesthat would be considered hazardous to humans. For example, the location,which may be a dead end of an inclined drillhole, a J-section drillhole,or peaks of a vertically undulating drillhole, may have no direct upward(e.g., toward the surface) path to a vertical portion of the drillhole.

A number of implementations have been described. Nevertheless, it willbe understood that various modifications may be made without departingfrom the spirit and scope of the disclosure. For example, exampleoperations, methods, or processes described herein may include moresteps or fewer steps than those described. Further, the steps in suchexample operations, methods, or processes may be performed in differentsuccessions than that described or illustrated in the figures.Accordingly, other implementations are within the scope of the followingclaims.

1. (canceled)
 2. A method, comprising: forming a drillhole that extendsinto the Earth from a terranean surface, the drillhole comprising anentry at least proximate the terranean surface, the drillhole comprisinga substantially vertical drillhole portion and a hazardous materialstorage drillhole portion that is coupled to the substantially verticaldrillhole portion and formed in a subterranean salt formation; moving astorage canister into the hazardous material storage drillhole portion,the storage canister sized to fit from the drillhole entry through thesubstantially vertical drillhole portion, and into the hazardousmaterial storage drillhole portion of the drillhole, the storagecanister comprising an inner cavity that encloses nuclear wastematerial; and positioning a seal in the drillhole to isolate thehazardous material storage drillhole portion of the drillhole from theentry of the drillhole.
 3. The method of claim 2, wherein the nuclearwaste material comprises plutonium.
 4. The method of claim 2, whereinthe subterranean salt formation comprises an impermeable formationlayer.
 5. The method of claim 4, wherein the impermeable formation layeris vertically isolated from a subterranean formation that comprisesmobile water.
 6. The method of claim 2, wherein the drillhole furthercomprises a substantially non-vertical drillhole portion.
 7. The methodof claim 6, wherein at least a portion of the substantially non-verticaldrillhole portion comprises the hazardous material storage drillholeportion.
 8. The method of claim 2, further comprising: installing acasing in at least a portion of the drillhole; and installing a cementlayer adjacent the casing.
 9. The method of claim 8, wherein installingthe casing comprises installing the casing in at least a portion of thesubstantially vertical drillhole portion.
 10. The method of claim 2,wherein positioning the seal comprises positioning the seal in thesubstantially vertical drillhole portion.
 11. The method of claim 2,wherein the storage canister comprises a first storage canister, themethod further comprising: moving a second storage canister into thehazardous material storage drillhole portion adjacent the first storagecanister, the second storage canister sized to fit from the drillholeentry through the substantially vertical drillhole portion, and into thehazardous material storage drillhole portion of the drillhole, thesecond storage canister comprising an inner cavity that encloses nuclearwaste material.
 12. The method of claim 2, wherein the nuclear wastematerial comprises military nuclear waste material.
 13. The method ofclaim 12, wherein the subterranean salt formation comprises animpermeable formation layer.
 14. The method of claim 13, wherein theimpermeable formation layer is vertically isolated from a subterraneanformation that comprises mobile water.
 15. The method of claim 12,wherein the drillhole further comprises a substantially non-verticaldrillhole portion.
 16. The method of claim 15, wherein at least aportion of the substantially non-vertical drillhole portion comprisesthe hazardous material storage drillhole portion.
 17. The method ofclaim 9, wherein the nuclear waste material comprises military nuclearwaste material.
 18. The method of claim 17, wherein the storage canistercomprises a first storage canister, the repository further comprising: asecond storage canister positioned in the hazardous material storagedrillhole portion adjacent the first storage canister, the secondstorage canister sized to fit from the drillhole entry through thesubstantially vertical drillhole portion, and into the hazardousmaterial storage drillhole portion of the drillhole, the second storagecanister comprising an inner cavity sized to enclose military nuclearwaste material.
 19. The method of claim 12, further comprising:installing a casing in at least a portion of the drillhole; andinstalling a cement layer adjacent the casing.
 20. The method of claim19, wherein the subterranean salt formation comprises an impermeableformation layer, and the impermeable formation layer is verticallyisolated from a subterranean formation that comprises mobile water. 21.The method of claim 20, wherein the drillhole further comprises asubstantially non-vertical drillhole portion, and at least a portion ofthe substantially non-vertical drillhole portion comprises the hazardousmaterial storage drillhole portion.